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LSU Doctoral Dissertations Graduate School

2013 The Theory and the of Quantum Schubert Cells Joel Benjamin Geiger Louisiana State University and Agricultural and Mechanical College

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Recommended Citation Geiger, Joel Benjamin, "The and the Representation Theory of Quantum Schubert Cells" (2013). LSU Doctoral Dissertations. 1967. https://digitalcommons.lsu.edu/gradschool_dissertations/1967

This Dissertation is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Doctoral Dissertations by an authorized graduate school editor of LSU Digital Commons. For more information, please [email protected]. The Ring Theory and the Representation Theory of Quantum Schubert Cells

A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy in The Department of Mathematics

by Joel B. Geiger B.S., Nicholls State University, 2007 M.S., Louisiana State University, 2010 August 2013 To my best friend and dear wife Laura Rider: Thank you for the effort you put into our relationship, thank you for being supportive of my work and not taking away from my time to do it, and most of all thank you for never failing to be fascinating in your words and your deeds. I dedicate this dissertation to you.

ii Acknowledgments

First and foremost I am thankful to my advisor Milen Yakimov, Thank you for guidance, both mathematical and otherwise, thank you for being a terrific teacher, and thank you for being an active advisor, for urging me to be involved in the mathematical community. You introduced me to the world of mathematical research. You sent me off to my first conference, invited me to my first talk, worked closely with me, and answered my same questions over and over without fail. You were invaluable in my formation as I transitioned through my apprenticeship with you. Perhaps one day I will be in a position to reciprocate your kindnesses. I am thankful to Ken Goodearl for helpful comments regarding the main results in this thesis and for coauthoring, with Ken Brown, my favorite mathematical text. I wish to thank my committee members — Daniel Sage, Pramod Achar, Daniel Cohen, Leonard Richardson, Bill Hofffman, and Milen Yakimov — for allowing me to to run over- time in my defense! A special thanks to Pramod Achar for warning me of the ensuing snake battle. In my time at LSU, I was fortunate to have been supported by several grants, both travel and otherwise. I wish to thank Frank Neubrander and James Madden for supporting me in my NSF-GK12 appointment. I thank Bill Hoffman for supporting my summer research via NSA grant H98230-11-1-0160. Finally, I thank the NSF VIGRE PIs, Gestur Olafsson, Larry Smolinsky, and Mark Davidson for supporting my studies and travel with NSF grant DMS-0739382. Thank you to my wonderful math teachers from Nicholls State University — Scott Beslin, Brain Heck, Van Ma, and Don Bardwell. You sparked my interest in mathematics and won me over from my more lucrative studies in chemistry! I still use a combination of you as a model for what I think a good teacher should be. Thank you James Oxley for your helpful comments and suggestions regarding my teaching. As a top researcher in your field and and terrific teacher, you were another great role model for me. Thank you to the dedicated professors from whom I have taken classes, including all my committee members, William Adkins, Robert Perlis, Pallavi Dani, Neal Stoltzfus, Boris Rubin, and Richard Litherland. Thank you all my wonderful friends at LSU, whom are too many to name (If you think I have not considered you, you are wrong!). When I think back on my interactions with you, it will always be fondly. A special thanks to Greg Muller for being a good friend and for always being up to the task of talking math with me. You were like a second advisor, and I would wish you luck in your future, but given your talent and drive that might be somewhat of an insult! Thank you to all my friends around the world, many of whom I met at conferences. Thank you to my letter writers and to all who helped me find employment! Thank you to my dear parents Rose and Benjamin Geiger and my extended family for teaching me so much including the importance of education in relation to my career prospects. I attribute much of my success to my the good fortune in having you in my life, Mom and

iii Dad. Thank you to my two brothers and sister, Kyle, Brett, and Jennifer for putting up with a tyrant of an older brother like me! I also would like to thank my inlaws for being supportive of and for showing sincere interest in my endeavors. I extend a very special thanks to my best friend and wife Laura Rider for sharing your life, your knowledge, your successes, and your love with me and for letting me share my life with you. You taught me everything I know about perverse sheaves! Lastly, I wish to thank thank my beautiful family, Laura, Ian, and Jenna, as a whole. This small margin cannot contain the words to express the emotions you bring me. C’est en vous que je trouve mon sens, mon raison d’ˆetre.

iv Table of Contents

Acknowledgments ...... iii

List of Figures ...... vii

Notational Conventions...... ix

Abstract ...... x

Introduction ...... 1

Chapter 1: Lie theory background ...... 8 1.1 preliminaries ...... 8 1.2 The Cartan–Killing Form ...... 13 1.3 Classification of semisimple Lie algebras ...... 15 1.4 Root systems and Cartan matrices ...... 18

Chapter 2: Stratifying SpecR ...... 22 2.1 Goodearl–Letzter Stratification ...... 22 2.2 Cauchon diagrams ...... 24

Chapter 3: Constructing quantum Schubert cells ...... 29 3.1 Quantized Universal Enveloping Algebras ...... 29 3.2 Quantized Coordinate Rings ...... 36 3.3 Weyl combinatorics ...... 39 3.4 Explicit construction ...... 43

Chapter 4: Ring vs representation theoretic approaches to SpecU −[w] ...... 48 4.1 Cauchon’s effacement des d´erivations ...... 48 4.2 The prime spectrum of U −[w] via Demazure modules ...... 56

Chapter 5: Main results and implications ...... 62 5.1 Statement of main result ...... 62 5.1.1 Leading terms of quantum minors ...... 64 5.1.2 Proof of Theorem 5.1.4 ...... 68 5.2 Unification of the two approaches to H-SpecU −[w] ...... 72 5.2.1 Solutions of two questions of Cauchon and M´eriaux ...... 72 5.2.2 Proof of Theorem 5.2.1 ...... 77 5.2.3 Proof of Theorem 5.2.5 (i) ...... 79 5.2.4 Proof of Theorem 5.2.5 (ii) ...... 81

v References ...... 86

Appendix ...... 89

Vita ...... 98

vi List of Figures

2.1 The fourteen Cauchon diagrams of Rq[M2] with missing inclusions ...... 28 A.1 Relation diagram for i = 121321 ...... 92

A.2 Relation diagram for i = 121321 ...... 92

A.3 Relation diagram for i = 123121 ...... 92

A.4 Relation diagram for i = 123212 ...... 93

A.5 Relation diagram for i = 132312 ...... 93

A.6 Relation diagram for i = 132132 ...... 93

A.7 Relation diagram for i = 212321 ...... 94

A.8 Relation diagram for i = 213231 ...... 94

A.9 Relation diagram for i = 213213 ...... 94

A.10 Relation diagram for i = 231213 ...... 95

A.11 Relation diagram for i = 231231 ...... 95

A.12 Relation diagram for i = 232123 ...... 95

A.13 Relation diagram for i = 312132 ...... 96

A.14 Relation diagram for i = 312312 ...... 96

A.15 Relation diagram for i = 321232 ...... 96

A.16 Relation diagram for i = 321323 ...... 97

A.17 Relation diagram for i = 323123 ...... 97

vii viii Notational Conventions

Symbol Definition k, q ∈ k× arbitrary field and a nonzero, non root of unity g ⊇ h complex semisimple Lie algebra and choice of Cartan subalgebra

∨ ∆, ∆ , ∆+ root system of rank r, coroots, and positive roots

∨ Π = {α1, . . . , αr}, α choice of simple roots, coroot of α ⟨−, −⟩ symmetric bilinear form on RΠ normalized to ⟨α, α⟩ = 2 for short roots

Q ⊂ P , Q≥0, P≥0 roots , weight lattice, dominant integral part of root and weight lattice

I ⊂ Π, QI parabolic subset of simple roots and corresponding root lattice

{ϖ1, . . . , ϖr} fundamental weights of g ∈ w W , si1 , . . . , sir Weyl group of g and generating simple reflections i, w, W ≤w reduced word for w, reduced decomposition of w, Bruhat order interval B , Ti1 ,...,Tir braid group associated to the W and generators

Uq(g) quantized universal enveloping algebra of g

M semisimple category of finite dimensional integrable Uq(g)-modules

V (λ) ∈ M irreducible highest weight Uq(g)- of highest weight λ U −[w], SpecU −[w] quantum Schubert cell algebra associated to w, n.c. prime spectrum of U −[w]

H, H-SpecU −[w] algebraic torus (k×)r and the H-prime spectrum of U −[w]

φ : U −[w] ,→ U −[w] Cauchon’s deleting derivations map to the Cauchon quantum affine space

G connected, simply connected, algebraic group G with Lie algebra g

Rq[G] ⊃ R quantized coordinate ring of G, dominant part of Rq[G]

⊃ 0 Rw Rw localization of R, Cartan-invariant subalgebra of Rw (under right hit) 0 ↠ − − ϕw : Rw U [w] Yakimov’s surjection from the representation theoretic approach to U [w]

ix Abstract

In recent years the quantum Schubert cell algebras, introduced by Lusztig and De Concini–

Kac, and Procesi, have garnered much interest as this versatile class of objects are furtive testing grounds for noncommutative . We unify the two main approaches to analyzing the structure of the torus-invariant prime spectra of quantum Schubert cell algebras, a ring theoretic one via Cauchon’s deleting derivations and a representation the- oretic characterization of Yakimov via Demazure modules. As a result one can combine the strengths of the two approaches. In unifying the theories, we resolve two questions of

Cauchon and M´eriaux, one of which involves the Cauchon diagram containment problem.

Moreover, we discover explicit quantum-minor formulas for the final generators arising from iterating the deleting derivation method on any quantum Schubert cell algebras. These for- mulas will play a large role in subsequent research. Lastly, we provide an independent and elegant proof of the Cauchon–M´eriauxclassification. The main results in this thesis appear in [GY] and are joint with Milen Yakimov.

x Introduction

Approximately twenty years ago, Joseph [Jos94,Jos95] and Hodges–Levasseur–Toro [HLT97]

obtained a number of important results on them the spectra of quantum groups for generic

parameter q,

One of their main goals was to understand these spectra in terms of symplectic foliations

in an attempt to extend the orbit method [Dix96] to more general classes of noncommu-

tative algebras. This lead to the study of various quantum analogs of universal enveloping

algebras of solvable Lie algebras. The quantum Schubert cell algebras U −[w], defined by De

Concini–Kac–Procesi [DCKP95] and Lusztig [Lus93], comprise a large and versatile such

class algebras. For every simple Lie algebra g and an element w, one constructs U −[w],

which is a deformation of the universal enveloping algebra U(n− ∩ w(n+)) and a subalgebra

of the quantized universal enveloping algebra Uq(g) Alternatively, we can view the algebra U −[w] as a deformation of the coordinate ring of the Schubert cell corresponding to w of the

full flag variety of g equipped with the standard Poisson structure [GY09]. These algebras

played important roles in many different contexts in recent years such as the study of coideal

subalgebras of Uq(b−) and Uq(g)[HS,HK12] and quantum cluster algebras [GLS]. There are two very different approaches to the study of the spectra of U −[w]. One is purely ring theoretic and is based on the Cauchon procedure of deleting derivations [Cau03a]. The

second is a representation theoretic one and builds on the above mentioned methods of

Joseph, Hodges, Levasseur, and Toro [Jos95, HLT97]. Each of these methods has a number

of advantages over the other, and relating them was an important open problem with many

potential applications. Previously there were no connections between them even for special

cases of the algebras U −[w], such as the algebras of quantum matrices.

1 In this paper we unify the ring theoretic and the representation theoretic approaches

to the study of SpecU −[w]. Furthermore, we resolve several other open problems on the deleting derivation procedure and the spectra of U −[w], two being questions posed by

Cauchon and M´eriaux[MC10]. Before we proceed with the statements of these results, we

need to introduce some additional background.

There is a canonical action of the torus H = (k∗)×r on U −[w] by algebra automorphisms, where k is the base field and r is the rank of g. By a general stratification result of Goodearl and Letzter [GL00], one has a partition

⊔ − − ∈ H − SpecU [w] = SpecI U [w] over I -SpecU [w] I

Here H-SpecU −[w] denotes the set of H-invariant prime ideals. By two general results of

[GL00] H-SpecU −[w] is finite and each stratum { } ∩ − ∈ − | SpecI U [w] = L SpecU [w] t(L) = I t∈H

is homeomorphic to the spectrum of a (commutative) Laurent . The problem

of the description of the Zariski topology of SpecU −[w], however, is wide open.

The Cauchon method of deleting derivations is a multi-stage recursive procedure [Cau03a]

beginning with an iterated Ore extension A of length l (of a certain general type) equipped

with a compatible H-action and ending with a quantum affine space algebra A with a H- action. Cauchon constructed in [Cau03a] a set-theoretic embedding of SpecA into SpecA. It restricts to a set-theoretic embedding H-SpecA,→ H-SpecA. The H-invariant prime ideals

of A are then parametrized by some of the subsets of [1, l], called Cauchon diagrams. The H- prime ideal of A corresponding to a Cauchon diagram D ⊆ [1, l] will be denoted by JD. The problem of determining which subsets of [1, l] arise in this way (i.e., are Cauchon diagrams), is the essence of the method and is very difficult for each particular class of algebras. It

2 was solved for the algebras of quantum matrices by Cauchon [Cau03a] and for all algebras

U −[w] by Cauchon and M´eriaux[MC10]. To state the latter result, we denote the set of

simple roots of g by ∆ and the corresponding simple reflections of W by sα, α ∈ ∆. A

word i = (α1, . . . , αl) in the alphabet ∆ will be called a reduced word for w if sα1 . . . sαl is a reduced expression of w. Each reduced word i for w gives rise to a presentation of

U −[w] as an iterated Ore extension of length l. The subsets of [1, l] are index sets for the { } subwords of i by the assignment j1 < . . . < jn . . .(αj1 , . . . , αjn ). We will denote by ≤ the (strong) Bruhat order on W and set W ≤w = {y ∈ W | y ≤ w}. For each y ∈ W ≤w

there exists a unique left positive subword of i corresponding to y (see Section 2.2 for its D+ definition and details on Weyl group combinatorics). Its index set will be denoted by i (y). The Cauchon–M´eriauxclassification theorem states the following:

For all Weyl group elements w ∈ W and reduced words i for w, consider the presentation of U −[w] as an iterated Ore extension correspodning to i. The Cauchon diagrams of the

H − D+ ∈ ≤w -prime ideals of U [w] are precisely the index sets i (y) for y W . The representation theoretic approach [Yak10] to the spectra SpecU −[w] relies on a family

H w − w of surjective -equivariant antihomomorphisms ϕw : R0 . . .U [w], where R0 are certain

w quotients of subalgebras of the quantum groups Rq[G]. The algebras R0 were introduced by Joseph [Jos95] as quantizations of the coordinate rings of w-translates of the open Schubert cell of the flag variety of g, see Section 4.2 for details. Via these maps one can transfer

back and forward questions on the spectra of U −[w] to questions on the spectra of quan- tum function algebras. The latter can be approached via representation theoretic methods, building on the works of Joseph [Jos94,Jos95], Gorelik [Gor00], and Hodges–Levasseur–Toro

[HLT97]. This leads to an explicit picture for H-SpecU −[w]. First, the H-invariant prime

− ≤w ≤w ideals of U [w] are parametrized by W , and the ideal Iw(y) corresponding to y ∈ W

is explicitly given in terms of Demazure modules using the maps ϕw, see 4.2.2 for a precise

3 − statement. Second, each of the strata SpecIw(y)U [w] consists of ideals constructed by con- − tractions from localizations of U [w]/Iw(y) by explicit small multiplicative sets of normal elements.

Each of the above two methods has many advantages over the other. Using the representa-

tion theoretic approach, it was proved that all ideals Iw(y) are polynormal, it was established that U −[w] are catenary and satisfy Tauvel’s height formula, the containment problem for

− ≤w H-SpecU [w] = {Iw(y) | y ∈ W } was solved, and theorems for separation of variables for U −[w] were established (see [Yak10,Yakb,Yakc]). In the special case of the algebras of quan- tum matrices, catenarity and ideal containment was proved earlier [Cau03b, Lau07] within

the framework of the ring theoretic approach (though with more complicated arguments),

but there was no progress on polynormality or proofs of these results for more general U −[w]

algebras. On the other hand using the ring theoretic approach, it was proved that for all

− − H-primes JD of U [w] the factor U [w]/JD always has a localization that is a quantum torus, its center (which is closely related to the structure of the stratum Spec U −[w]) was JD described, and in the case of quantum matrices H-primes were related to total positivity (see

[Cau03a,BCL,GLL11]).

Our first result resolves Question 5.3.3 of Cauchon and M´eriaux[MC10] and unifies the

two approaches to H-SpecU −[w]

Theorem 0.0.1. Let k be an arbitrary base field, q ∈ k∗ not a root of unity, g a simple

Lie algebra, w an element of the Weyl group of g, and i a reduced word for w. Consider

the presentation of the quantum Schubert cell algebra U −[w] as an iterated Ore extension

corresponding to i.

4 Then for all Weyl group elements y ≤ w the Cauchon diagram of the H-prime ideal Iw(y) − D+ of U [w] (from the representation theoretic approach from 4.2.2 (i)) is equal to i (y), the index set of the left positive subword of i whose total product is y.

Thus the H-prime ideals of U −[w] from the representation theoretic approach are related

to the ideals JD from the ring theoretic approach via

Iw(y) = JD+ . i (y)

Furthermore, we prove a theorem that explicitly describes the behavior of the represen-

− tation theoretic ideals Iw(y) of U [w] in each stage of the Cauchon deleting derivation procedure. This appears in 5.2.5 below and will not be stated in the introduction since it

requires additional background.

With the help of 0.0.1, one can now combine the strengths of the two approaches to the

spectra of the quantum Schubert cell algebras. We expect that the combination of the two

methods will lead to substantial progress in the study of the topology of SpecU −[w]. We use

0.0.1 and previous results of the second author to resolve Question 5.3.2 of Cauchon and M´eriaux[MC10],

thereby solving the containment problem for the ideals

≤w {JD+ | y ∈ W } i (y)

of the classification of [MC10].

Theorem 0.0.2. In the setting of Theorem 0.0.1, the map

≤w − W . . . H-SpecU [w] given by y . . .JD+ i (y)

is an isomorphism of posets with respect to the (strong) Bruhat order and the inclusion order

on ideals.

Finally, 0.0.1 also gives a new, independent proof of the Cauchon–Meriaux classification

[MC10] described above. (The proof of 0.0.1 does not use results from [MC10].)

5 Let us return to the general case of Cauchon’s method of deleting derivations. It relates

the prime ideals of an initial iterated Ore extension A to the prime ideals of the final algebra

A, the Cauchon quantum affine space algebra associated to A. In order to study these ideals, one needs an explicit description of A as a subalgebra of the ring of fractions of A. We obtain

such for all algebras U −[w], establishing yet another relationship between the two approaches

− to the structure of the algebras U [w]. Given a reduced word i = (α1, . . . , αl) for w, define a successor function s: [1, l] ⊔ {∞} . . .[1, l] ⊔ {∞} by

s(j) = min{k | k > j, αk = αj}, if ∃k > j such that αk = αj, s(j) = ∞, otherwise.

− For j ∈ [1, l] denote by ∆i,j ∈ U [w] the element obtained by evaluating the quantum minor

corresponding to the fundamental weight ϖαj and the Weyl group elements sα1 . . . sαj−1 , w ∈ W on the R- Rw corresponding to w. We refer to Section 4.2 and Section 5.1

for details and the description of these elements in the framework of the antiisomorphisms

w − ϕw : R0 . . .U [w].

Theorem 0.0.3. In the setting of 0.0.1, for all Weyl group elements w and reduced words

i = (α1, . . . , αl) for w, the generators x1,..., xl of the corresponding Cauchon quantum affine space algebras are given by   (q−1 − q )−1∆−1 ∆ , if s(j) ≠ ∞ αj αj i,s(j) i,j xj =  (q−1 − q )−1∆ , if s(j) = ∞ αj αj i,j

∈ k∗ for the standard powers qαj of q, see Section 3.1.

This theorem establishes a connection between the initial cluster for the cluster algebra structure on U −[w] of Geiß–Leclerc–Schr¨oerand Cauchon’s method of deleting derivations.

We will present a deeper study of this in a forthcoming publication. 0.0.3 is also an important

6 ingredient in a very recent proof [Yakd] of the second author of the Andruskiewitsch–Dumas

conjecture [AD08].

The paper is organized as follows. Section 3.4 contains background on the quantum Schu-

bert cell algebras and the representation theoretic and ring theoretic approaches to the study

of their spectra. Section 5.1.2 contains the proof of 0.0.3. Theorems 0.0.1 and 0.0.2 are proved

in Section 5.2, where we also establish a theorem describing the behavior of the ideals Iw(y) under the iterations of the deleting derivation procedure.

7 Chapter 1 Lie theory background

1.1 Lie algebra preliminaries

Throughout this introduction k will denote a field of characteristic 0. All vector spaces

will be over k and all k-algebras will be unital and associative. We begin by constructing several important algebras associated to V . Let V be a and T iV := V ⊗i. We

adopt the convention T 0V := k.

Definition 1.1.1. The T (V ) of V is the vector space

⊕∞ T iV i=0

with defined by the concatenation isomorphism T mV ⊗ T nV . . .T m+nV.

By this definition it is clear that T (V ) is a graded algebra if we take the k-th graded piece

to be T kV .

Definition 1.1.2. The symmetric algebra S(V ) is the quotient T (V )/I, where I is the ideal

of T (V ) generated by x ⊗ y − y ⊗ x for all x and y in V .

Here and after we identify V with its canonical image in T (V ).

∧ Definition 1.1.3. The (V ) is the quotient T (V )/I, where I is the ideal

of T (V ) generated by x ⊗ x for all x in V .

The exterior algebra inherits a natural grading from T (V ). We denote its k-th graded ∧ piece by k(V ).

8 Definition 1.1.4. A Lie algebra is a vector space g endowed with an alternating [−, −]: g × g . . .g satisfying the Jacobi identity

[[x, y], z] + [[y, z], x] + [[z, x], y] = 0

The Jacobi identity fulfills the role of an associativity condition for the (Lie) bracket [−, −], which is a nonassociative product. If the bracket is 0, then the Lie algebra is called abelian.

We record the canonical example of a lie algebra. If A is any associative k-algebra, we may define [x, y] = xy − yx, the commutator bracket. Then, A equipped with this product is a

Lie algebra. This procedure defines a functor Lie(−) from the category associative algebras to the category of Lie algebras. Given any vector space V , we can form the Lie algebra

Lie(End(V )). We will simplify notation and write gl(V ) to denote the aforementioned Lie algebra. A Lie algebra map φ : g . . .g′ is a preserving the bracket, that is,

satisfying φ([x, y]) = [φ(x), φ(y)]. If a subspace h of g is closed under the bracket, then it is

Lie subalgebra. To encode this we write h ≤ g. If h satisfies the stronger condition [g, h] ⊂ h,

then we write h  g and call h an ideal. We should note that if a ⊴ g is an ideal and b ≤ g is

a subalgebra, then a + b is a subalgebra. An ideal is called proper if it is not the zero ideal

or g itself. A nonabelian Lie algebra with no proper ideals is called simple. Given two Lie

algebras g and g′, we may construct the coproduct g ⊕ g′. It is the usual categorical vector

space coproduct with bracket

[(x1, y1), (x2, y2)] := ([x1, y1] , [x2, y2]) .

If g = a ⊕ i for some subalgebra a and some ideal i then we write g = a ⋉ i. If a is also an

ideal, then g is a coproduct a ⊕ i, which is also a product a × i. For any derivation action

φ : a −→ Der(i) mapping a to φa, we can give a ⋉ i a Lie algebra structure by defining

− [(x1, y1), (x2, y2)] := ([x1, y1] , [x2, y2] + φx1 (y2) φy1 (x2)) .

9 A representation of g or a g-module (when k is merely a ) is a k-module

M and a bilinear product g ⊗k M −→ M satisfying [x, y] m = x(ym) − y(xm). Equivalently,

a representation of g is a Lie algebra morphism φ : g . . . End(M). We will denote the

category of all g modules by g-Mod. Define a map

ad : g −→ End(g)

by x 7→ adx, where adx(−) = [x, −]. This is called the adjoint representation of g. The kernel of ad is called the center of g denoted by z(g). It is clear by construction that kerad = g if and only if g is abelian. Let R be a (not necessarily associative) . A derivation ∂ is an endomorphism of R satisfying

∂(ab) = ∂(a)b + (−1)|a|a∂(b), where |a| denotes the degree of a. Let Der(R) denote the derivation algebra of R. The Jacobi identity and alternating property of the bracket give immediately that adx : g . . .g is in

Der(g); here we consider g with the trivial grading, that is, g is concentrated in degree zero.

A Lie algebra derivation ∂ is called an inner derivation if ∂ = adx for some x in g. Although Lie algebras are not associative in general, we can construct an by forcing the bracket to be a commutator. This construction gives much insight into the representation theory of g. The universal enveloping algebra U(g) of g is the quotient

T (g)/I, where I is the ideal of T (V ) generated by [x, y] − x ⊗ y − y ⊗ x for all x and y in g.

By construction we see that a g-module is precisely a module for U(g). More precisely, for every k-algebra A there is a natural isomorphism

∼ Homk-Alg(U(g),A) = HomLie(g, Lie(A)).

The statement follows from setting A = End(g).

10 We briefly give an alternate definition of a Lie algebra in terms of a construction of

Chevalley and Eilenberg [CE48]. Given a vector space V with an alternating bilinear map

[−, −]: V × V → V , we can construct the sequence of vector spaces

∧n+1 ∧n CV := ... −→ (V ) −→ (V ) −→ ... −→ V −→ k −→ 0 ∧ ∧ with maps ∂ : n+1(V ) −→ n(V ) defined by 1 ∑ ∂(x ∧ · · · ∧ x ) = (−1)i+j+1 [x , x ] ∧ x ∧ · · · ∧ xb ∧ · · · ∧ xb ∧ · · · ∧ x 1 n+1 n + 1 i j 1 i j n+1 i

|ω1| ∂(ω1 ∧ ω2) = ∂ω1 ∧ ω2 + (−1) ω1 ∧ ∂ω2,

where |x| denotes the degree of x.

Lemma 1.1.5. Suppose g is a vector space equipped with such a bracket. The sequence Cg

is a chain complex if and only if g is a Lie algebra. ∧ Proof. Consider the first nontrivial case ∂2 : 3(g) −→ g. A direct computation yields 1 ∂2(x ∧ y ∧ z) = ∂([x, y] ∧ z − [x, z] ∧ y + [y, z] ∧ x) 3 1 = [[x, y], z] − [[x, z], y] + [[y, z], x] 6 1 = [[x, y], z] + [[z, x], y] + [[y, z], x] 6 which is 0 if and only if [−, −] satisfies Jacobi. The general case follows from applying ∂2 to

x1 ∧ · · · ∧ xn = x1 ∧ ω to obtain

∂ [∂(x1 ∧ ω)] = ∂ [∂(x1) ∧ ω − x1 ∧ ∂(ω)] [ ] 2 2 = ∂ (x1) ∧ ω + ∂(x1) ∧ ∂(ω) − ∂(x1) ∧ ∂(ω) − x1 ∧ ∂ (ω)

2 2 = ∂ (x1) ∧ ω + x1 ∧ ∂ (ω)

2 = x1 ∧ ∂ (ω),

11 which is zero by the inductive hypothesis.

By the previous lemma, we may define a Lie algebra to be a vector space g with a linear ∧ 2 map φ : (g) −→ g such that (Cg, φ) is a chain complex. The Hn(g) :=

ker ∂n+1/ im ∂n is called the n-th homology group of g. If M is any g-module we apply the

functor Hom(−,M) to Cg to obtain the analogously defined cohomology groups H∗(g,M)

with values in M.

The following collections of Lie algebras will play a major role in Lie algebra representation

theory. A Lie algebra g is called solvable if the derived series

g(0) ⊇ g(1) ⊇ ... g(n) ⊇ ... [ ] terminates. Here g(0) = g and g(i+1) = g(i), g(i) . A less restrictive property than solvability

is nilpotence. A Lie algebra g is nilpotent if the lower central series

g0 ⊇ g1 ⊇ ... gn ⊇ ...

terminates. Here g0 = g and gi+1 = [g, gi].

Proposition 1.1.6. Consider the short exact sequence of Lie algebras

0. a. i g. π b. 0.

We have g is solvable (resp. nilpotent) if and only if a and b are solvable (resp. nilpotent).

If g has basis {X1,...Xn}, then the Lie algebra structure is completely determined by the

k ∈ k structure constants— elements ci,j such that ∑n k k [Xi,Xj] = ci,jX k=1 Proposition 1.1.7. Suppose a and b are solvable (resp. nilpotent) ideals of g, then a + b is

a solvable (resp. nilpotent) ideal.

12 We define rad(g) the radical of a Lie algebra to be the sum of all the solvable ideals of g.

Similarly, the nilradical nil(g) is the sum of all the nilpotent ideals. We call g semisimple if rad(g) = 0 and reductive if g = s ⊕ a where s is a semisimple Lie algebra and a is an abelian

Lie algebra.

Reductive Lie algebras are generalizations of semisimple Lie algebras in the sense that every semisimple Lie algebra is reductive. This follows immediately from the straightforward observation

0 = rad(s ⊕ a) ⊇ z(s ⊕ a) = a.

Fixing g gives a non-split (in general) exact sequence

0. nil(.g) i . g. π b. 0. and a split exact sequence

0. rad.(g) i . g. π l. 0..

By construction we see that l is semisimple. This subalgebra is called the levi subalgebra and the decomposition g = l ⋉ rad(g) is called the levi decomposition.

Example 1.1.8. Let sl and t be the subalgebras of gl = gl(V ) given by traceless endomor- phisms and scalar multiples of the identity transformation, respectively. We have gl = sl ⊕ t.

1.2 The Cartan–Killing Form

Let ⟨−, −⟩ be the usual inner product on gl(V ) given by

⟨A, B⟩ = tr AB.

For any lie algebra representation φ : g . . .gl(V ), we define the symmetric bilinear form

⟨x, y⟩φ = ⟨φx, φy⟩ = tr φxφy.

13 When φ = ad we obtain the Cartan–Killing form, which we will denote by ⟨−, −⟩ in unam- biguous circumstances. For any subspace a ⊂ g, we define the orthogonal complement a⊥ to be all the elements of g whose bracket with every element of a is 0. In symbols

a⊥ = {x ∈ g | ⟨x, a⟩ = 0} .

Such a product is called nondegenerate on g if g⊥ = 0.

Proposition 1.2.1. The Killing form is invariant in the sense that

⟨[X,Y ],Z⟩ = ⟨X, [Y,Z]⟩.

Proof. A straightforward computation yields

⟨[X,Y ],Z⟩ = tr ad[X,Y ] adZ

= tr [adX , adY ] adZ

= tr adX adY tr adZ − tr adY adX adZ

= tr adX adY adZ − tr adX adY adZ

= tr adX [adY , adZ ]

= tr adX ad[Y,Z] = ⟨X, [Y,Z]⟩.

The equality ad[A,B] = [adA, adB] follows immediately from the Jacobi identity.

An immediate corollary is if a is an ideal, then so is a⊥.

Proposition 1.2.2. If a is an ideal of g, then the Cartan form of a coincides with the Cartan

form on g restricted to a.

Proof. We have ada(g) ⊂ a. Thus, we choose a basis for g so that for all z ∈ a  

ad 0  z a  adz =   0 0

14 We then have that for all x, y ∈ a      

adx ∗ ady ∗ adx ady 0 ⟨x, y⟩ = tr  a   a  = tr  a a  = ⟨x, y⟩ a 0 0 0 0 0 0

Proposition 1.2.3. A Lie algebra is semisimple if and only if it is a direct sum of simples.

Proof. Suppose a is a proper ideal of g, then

g = a ⊕ a⊥

If a′ is an ideal of a, then a′ is also an ideal in g. This is clear since [ ] [g, a′] = a ⊕ a⊥, a′ [ ] and a⊥, a′ vanishes.

Corollary 1.2.4. The lie algebra g is semisimple if and only if g = [g, g]

Theorem 1.2.5 (Cartan’s Criterion). The Killing form is nondegenrate on g if and only if g

is semisimple.

1.3 Classification of semisimple Lie algebras

A linear map φ ∈ End(g) is called diagonalizable or semisimple if g has a basis consisting

of eigenvectors v1, v2, . . . , vn of φ. This is equivalent to g being semisimple as a k[φ]-module.

g = kv1 ⊕ kv2 ⊕ ... ⊕ kvn

An element x ∈ g is called semisimple with respect to a representation φ if φx is a semisimple endomorphism. Subablgebras of g consisting entirely of semisimple elements are often called toral.

15 Let S ⊂ g be a subset. The normalizer of S in g is the set stabilzer under the action of

the bracket, that is,

N(S) = {x ∈ g | [x, s] ∈ S for all s ∈ S}

By construction if S is a subalgebra of g, then N(S) it is the largest Lie subalgebra of g

containing S as an ideal. A nilpotent subalgebra h ≤ g that is self-normalizing (N(h) = h) is called a Cartan subalgebra. Since the center z(g) is a solvable ideal, it is clear that a semisimple Lie algebra is centerless. If k is algebraically closed, however, then Cartan subalgebras are maximal abelian, toral subalgebras.

A weight of is a 1-dimensional representation λ : . . .k. Thus, φ = 0. Moreover, g g [g,g] since semisimple Lie algebras coincide with their derived subalgebras, semisimple Lie algebras

have no nontrivial weights. Restricted to a Cartan, however, weights λ ∈ h∗ need not be zero.

Let φ : g . . .gl(V ) be a representation of a semisimple Lie algebra and let h be a maximal toral subalgebra of g. Then, φ(h) is a commutative subalgebra of gl(V ) consisting

∗ entirely of semisimple operators. Let λ ∈ h be a weight, and define the λ-weight space Vλ

with respect to φ (really φ ) to be h

Vλ = {v ∈ V | φh(v) = λ(h)v for all h ∈ h}

When φ is the adjoint representation restricted to h and α ∈ h∗ is a weight, the definition

becomes

gα = {v ∈ g | [h, v] = α(h)v for all h ∈ h}

Immediately, we see g0 = h, and by construction g is a semisimple k[adh]-module for all

h ∈ h. That is, ⊕ ⊕ g = g0 ⊕ gα = h ⊕ gα α∈∆ α∈∆

Elements in gα are called weight vectors. The nonzero weights α for which gα are nonzero are called roots of g. The set of roots of g is denoted by ∆.

16 It is clear that if g is nilpotent, then adx will be nilpotent for any x ∈ g since

g ⊇ [g, g] ⊇ [g, [g, g]] ⊇ ...

n terminating in n steps implies (adx) (y) = 0 for all x, y ∈ g. Engel’s theorem is the converse statement.

Theorem 1.3.1 (Engel). Let φ : g . . .gl(V ) be a n > 0 representation of g, and suppose φx is nilpotent for all x ∈ g, then there is a flag in V

0 = V0 ⊂ V1 ⊂ · · · ⊂ · · · ⊂ Vn = V

with φg(Vi) ⊂ Vi−1 for 1 ≤ i ≤ n.

Corollary 1.3.2. If adx is nilpotent for all x ∈ g, then g is nilpotent.

Proof. Applying Engel’s theorem to φ = ad, we have

adx1 adx2 ... adxn (g) = [x1, [x2, [xn, g]]] = 0

n+1 n for any xi ∈ g. Therefore, g = 0 since (in particular) g ⊂ g.

Two remarks: A nonzero nilpotent lie algebra g must have nonzero center since the last

gi nonzero in the derived series is contained within z(g). Also, ad : g . . . End(g) is a faithful representation when z(g) = 0, e.g., when g is semisimple. For centerless Lie algebras this observation proves the simplest case of

Theorem 1.3.3 (Ado’s Theorem). Any finite dimensional Lie algebra has a faithful finite dimensional representation. ⊕ ⊕ If g is semisimple and g = h α∈∆ gα is a root space decomposition, define the subal- gebras ⊕ n = gα, b = h ⊕ n α∈∆

17 Proposition 1.3.4. The subalgebras n are nilpotent and b are solvable.

We call g = n− ⊕ h ⊕ n+ a triangular decomposition. Triangular decompositions gives rise to a corresponding decompostion of universal enveloping algebras

U(g) = U(n−) ⊗ U(h) ⊗ U(n+)

1.4 Root systems and Cartan matrices

We have seen the definition of roots in the classification of semisimple Lie algebras. Collec-

tions of roots, however, are intrinsically important as the following classical theorem suggests:

Theorem 1.4.1. Simple Lie algebras are completely determined by the irreducible crystal- lographic root systems.

We proceed by defining root systems as abstract combinatorial objects.

Definition 1.4.2. Let V be a finite dimensional real vector space with standard inner prod-

uct ⟨−, −⟩. A (reduced, crystallographic) root system in V is a finite set ∆ of vectors such

that

(i) the R-span of ∆ is V

(ii) if rα ∈ ∆, then r = 1

∈ − ⟨ ⟩ ⟨ ⟩ (iii) for all αi, αj ∆, we have sαi (αj) = αj cijαi is in ∆, where cij = 2 αi, αj / αi, αi

(iv) each cij is an integer.

The invertible maps φ : V . . .V satisfying φ(∆) ⊂ ∆ form a finite subgroup O∆ of

Aut(V ). The subgroup of O∆ generated by all the reflections sαi is called the Weyl group

of ∆ and is denoted W∆. A root system ∆ in V is reducible if ∆ = Φ1 ⊔ Φ2, where Φi is a

root system for the nonempty Wi, and V = W1 ⊕ W2. An isomorphism of root systems is an

18 isomorphism of vector spaces φ : V . . .V ′ such that φ(∆) = ∆′ and the matrix entries

cij are preserved. The rank of a root system is the dimension of V

Definition 1.4.3. A generalized Cartan matrix is a matrix C = (cij) with integral entries such that

(i) all diagonal entries are 2

(ii) all other entries are not positive.

(iii) the matrix C is the product of a symmetric matrix and an invertible diagonal matrix

Generalized Cartan matrices with positive determinant are exactly Cartan matrices.

Theorem 1.4.4 (Serre). A Cartan matrix C uniquely determines a semisimple Lie algebra g isomorphic to k⟨f1, . . . , fn, h1, . . . , hn, e1, . . . , en⟩ modulo the following relations:

− (i) [h , h ] = 0 (iii) [h , e ] = c e (v) ad1 cij (e ) = 0 i j i j ij j ei j

(ii) [e , f ] = δ h (iv) [h , f ] = −c f − i j ij i i j ij j (vi) ad1 cij (f ) = 0 fi j

Recall the reflection sα adds a multiple of the root α to any root β. In particular,

sα(β + kα) = β − cijα − kα = β − (cij + k)α.

The Serre relations (v) and (vi) reflect the fact that for α ≠ β, the chain

β − sα, β − (s − 1)α, . . . , β + tα is comprised of all roots, where s, t are the largest integers such that β − sα and β + tα are roots. To see this, note that ei − ej is not a root so s = 1 and then t = −cij. Thus, ei + (1 − cij)ej is not a root, and we have the anologous property

− ad1 cij (e ) = 0. ei j

19 Since [x, y] equals xy − yx ∈ U(g), we obtain the following simple formula whose straight- forward proof we leave to the reader

∈ n Proposition 1.4.5. For all x, y g, the formula for adx(y) in U(g) is given by ( ) ∑n n adn(y) = xkyxn−k x k k=0

Corollary 1.4.6. In U(g) the Serre relations are

− ( ) 1∑cij 1 − c − − ij eke e1 cij k = 0, (i ≠ j) k i j i k=0 − ( ) 1∑cij 1 − c − − ij f kf f 1 cij k = 0, (i ≠ j) k i j i k=0

Example 1.4.7. Let C be the Cartan matrix define by    2 −1 C =   . −1 2

The corresponding Lie algebra is then isomorphic to sl3. The generators e1 and e2 satisfy the Serre relations

[e1, [e1, e2]] = 0 and [e2, [e2, e1]] = 0.

In U(sl3) the corresponding relations are

2 − 2 [e1, [e1, e2]] = e1e2 2e1e2e1 + e2e1 = 0

2 − 2 [e2, [e2, e1]] = e2e1 2e2e1e2 + e1e2 = 0

Quantized analogues of the Serre relations in U(g) will play a major role in the study of

+ the positive part Uq of the quantized universal enveloping algebra of g . We conclude this section with the following well-known classification which highlights the deep interplay between Lie theory and root system combinatorics.

20 Theorem 1.4.8. There are bijective correspondences between any two of the following:

(i) irreducible Cartan matrices

(ii) isomorphism classes of simple Lie algebras

(iii) isomorphism classes of irreducible root systems

(iv) connected Dynkin diagrams

Furthermore, the following exhaust all connected Dynkin diagrams

. ... An (n + 1) α1 α2 αn−1 αn . ... ⇒ Bn (2) α1 α2 αn−1 αn . ... ⇐ Cn (2) α1 α2 αn−1 αn

αn . ... Dn (4) α1 α2 αn−2 αn−1

α6 . E6 (3) α1 α2 α3 α4 α5

α7 . E7 (2) α1 α2 α3 α4 α5 α6

α8 . E8 (1) α1 α2 α3 α4 α5 α6 α7 . ⇒ F4 (1) α1 α2 α3 α4 . ⇛ G2 α1 α2

21 Chapter 2 Stratifying SpecR

2.1 Goodearl–Letzter Stratification

Following the notational conventions in [BG02], we outline a procedure of Goodearl and

Letzter for analyzing the prime R. First, we say a proper ideal P ◁ R is prime if the product of ideals IJ being in P implies I or J is contained in P . We will call P completely prime if it is prime with respect to ideals, that is, if P is prime is the usual sense of . Primes need not be completely prime. The zero ideal in Mn(R), for instance, is prime but not completely prime. If H is a group acting on R, we say the ideal I

is H-invariant (or an H-ideal) if h(I) = I for all h ∈ H, and we call a proper H-ideal P an

H-prime ideal of R if the product of two H-ideals being in P implies at least one is in P . We

pause to make the important observation that H-invariant prime ideals are H-primes, but

the converse is false. Note that H-prime need not be prime. For instance,

∏ n Example 2.1.1. Let A be any simple ring. Set R = 1 A, let G be the symmetric group on n symbols Sn, and suppose G acts on R by permuting coordinates. Clearly, 0 is an H-prime, but is clearly not prime.

If 0 is an H-prime in R, we shall call R and H-prime ring. Let us denote by H-SpecR the

collection of H-prime ideals of R. We can endow H-SpecR with the Zariski topology, but

this will not play a role in this paper.

The action of H on R induces an action on SpecR, thereby partitioning SpecR into H-

orbits. An investigation of spectra with respect to H-orbits, however, may be impractical

as there are often more H-orbits than H-primes. We take the simple example of a Manins

22 quantum plane

2 Rq[k ] = k⟨x, y | xy = qyx⟩

to observe the difference between H-primes and H-orbits. Let k be a field. The affine plane

H = k2 acts on this quantum affine space by

× 2 2 (k ) ↷ Rq[k ]

(s, t)· xiyj = sitjxiyj.

There are six H-orbits

(i) ⟨0⟩ (iii) ⟨x⟩ (v) ⟨x − α, y | α ∈ k×⟩

(ii) ⟨x, y⟩ (iv) ⟨y⟩ (vi) ⟨x, y − β | β ∈ k×⟩,

but only the first four are H-primes. With this in mind, we define (I : H) to be the largest

H-ideal contained in I, that is, ∩ (I : H) = t(I). t∈H

Observe the following: If P is prime, then (P : H) is an H-prime, and if P1 and P2 are in the

same H-orbit, then (P1 : H) = (P2 : H) Following Goodearl and Letzter [GL00], we can now define the H-stratum of SpecR corresponding to J defined by

{ ∈ | H } SpecJ R = P SpecR (P : ) = J

and, thus, refine the orbit-stratification of SpecR to the H-stratification ⊔ SpecR = SpecJ R, J

where the union is over all H-primes in R. In this theory H-primes parametrize the strata of

SpecR. Moreover, we have H-prime ideal containment represented in the following diagram.

23 {⟨x, y − β .| β ∈ k×⟩} {⟨x,. y⟩} {⟨x − α, y .| α ∈ k×⟩}

{⟨x.⟩} . {⟨y.⟩}

. {⟨0.⟩} .

≤ We will later observe that this poset is isomorphic to the initial Bruhat order interval W s1s2 .

By restricting a result of Lorenz [Lor] if H is an algebraic torus acting rationally (see [[BG02]

Section II.2.6]) on R, then each H-stratum is H-equivariantly isomorphic to a torus—Spec of

a commutative Laurent polynomial algebra. The H-stratification theory is especially powerful

when the number of H-strata is known to be finite, as will be our case.

2.2 Cauchon diagrams

We follow the conventions of Cauchon [Cau03a,Cau03b] and very briefly outline a proce-

dure for obtaining . We later require more precise statement; see Section 4.1 for a proper

treatment of Cauchon’s algorithm.

Let A be a unital k-algebra, let A[X; σ, δ] be an Ore extension of A, and let S = {Xn | n ∈ N}. The deleting derivation is a map ϕ : A . . .A[X; σ, δ]S−1 given by

∑∞ − −n a. (1 q.) δ.nσ−n(a)X−n; n=0 (n)q!

2 n−1 here (n)q! = (1)(1+q) ... (1+q +q +···+q ). Under appropriate hypotheses, for instance the ones in [Cau03a], this sum gives well-defined algebra map ϕ satisfying

Xϕ(a) = ϕ(σ(a))X for all a ∈ A.

24 By the universal property of Ore extensions, we therefore have a unique map φ extending ϕ and mapping Y to X. A[Y.; σ] . φ.

ϕ A. A[X; σ,. δ]S−1

The image of φ is the algebra obtained from the algorithm. Cauchon’s method is an process

(N+1) that starts with some iterated Ore extension R = k[X1][X2; σ2, δ2] ... [XN ; σN , δN ] and yields a sequence of algebras

(j) ∼ R = k[Y1] ... [Yj−1; σj−1, δj−1][Yj; τj] ... [YN ; τN ] terminating with R(2) = R — the Cauchon (quantum affine) space

To illustrate this method, we restrict to the case of quantum matrices. Let k be an alge- braically closed field (for simplicity), and assume q ∈ k× is not a root of unity. The k-algebra

2 of n×n quantum matrices A is given by n generators Xi,j (1 ≤ i, j ≤ n) subject to relations:

a b × If ( c d ) is a 2 2 submatrix of (Xi,j), then

ab = qba, ac = qca,

bd = qdb, cd = qdc,

bc = cb, and

da = ad + (q − q−1)bc.

Another realization of A due to Faddeev, Reshetikhin, and Takhtadzhyan [RTF89] is as the

−1 associative algebra over k[q, q ] generated by formal entries Xij of a matrix X such that

R(X ⊗ I)(I ⊗ X) = (I ⊗ X)(X ⊗ I)R holds for the R-matrix ∑ ∑ ∑ −1 −1 R = q eii ⊗ eii + eii ⊗ ejj + (q − q) eij ⊗ eji. 1≤i≤n 1≤i≠ j≤n 1≤j

25 We denote this algebra by Rq[Mn] and note that the name “quantum matrices” or quantized coordinate ring of matrices is warranted, though we will not justify it here. Furthermore, it is easy to generalize this construction to obtain m × n quantum matrices.

There is a natural action of the algebraic torus H = (k×)2n by k-algebra automorphisms on A given by

(α1, . . . , αn, β1, . . . , βn) · Xi,j = αiβjXi,j.

This induces an action on SpecRq[Mn]. It is well known that Rq[Mn] is an iterated Ore extension of k and thus is a noetherian . In fact, the automorphisms and derivations can be read off by the relations in Rq[Mn]. A result of Goodearl and Letzter [GL00] gives

that all H-primes of A are completely prime and that H-SpecRq[Mn] is finite. The deleting derivations algorithm gives rise to an inclusion of sets

. φ : Spec.Rq[Mn] SpecR.q[Mn],

where Rq[Mn] is given by the generators and relations of A, except that now the relation da = ad + (q − q−1)bc is modified to da = ad. Moreover, this map descends to an inclusion on the respective H-prime spectra. If we denote the generators of Rq[Mn] by X1,..., Xn2 ,

then it is well-known that the H-primes in the quantum affine space Rq[Mn] have the form { } 2 QD = Rq[Mn]⟨Xi | i ∈ D ⊂ 1, . . . , n ⟩.

Cauchon deduced the preimage of φ is a disjoint union of strata indexed by elementary

combinatorial objects called Cauchon diagrams, which also parametrize H-orbits of symplec-

tic leaves in Mn [GLL11] and restricted permutations, σ ∈ S2n such that −n+i ≤ σ(i) ≤ n+i for 1 ≤ i ≤ 2n [BCL].

Definition 2.2.1. The Cauchon diagram of J ∈ H-SpecRq[Mn] is the unique set D such

that φ(J) = QD.

26 For n × n quantum matrices, the governing relation

da − ad = (q − q−1)bc (2.2.1) implies that if d is in a completely prime ideal, then either b or c must be as well. With some small additional argument, we conclude that the Cauchon diagrams for Rq[Mn] can be described combinatorially by n × n boxes such that if one square is filled, then every square to its left or every square above it is also filled. Explicitly, the general theory of deleting derivations yields the decomposition ⊔ φ(SpecRq[M2]) = SpecDRq[M2], D where the union is over all Cauchon diagrams. Moreover, Cauchon proves [Cau03a] that the

−1 collections φ (SpecDRq[Mn]) coincide with the Goodearl–Letzter stratification of SpecRq[Mn]. In the spirit of a running example, we observe the poset of Cauchon diagrams, which does

not correspond to the poset of H-primes in Rq[Mn]. The Cauchon diagram poset is missing two H-prime inclusions, which we denote in Figure 2.2 by dashed lines. We will later observe

that the full poset of H-primes, with the dashed lines honestly included, is isomorphic to (for

≤ instance) the initial Bruhat order interval W s2s1s3s2 . This consideration from the basis for

[[MC10] Question 5.3.2], which is the simple Corollary 5.2.4 following from one of our main

theorems. We shall make this precise in subsequent sections.

27 . . 1 .2 . . 3 4

1 .2 1 .2 . 1 .2 1 .2 3 4 3 4 3 4 3 4

1 .2 1 .2 1 .2 1 .2 1 .2 3 4 3 4 3 4 3 4 3 4 .

. 1 .2 1 .2 1 .2 . 3 4 3 4 3 4

. . 1 .2 . 3 4

. . .

FIGURE 2.1. The fourteen Cauchon diagrams of Rq[M2] with missing inclusions

28 Chapter 3 Constructing quantum Schubert cells

3.1 Quantized Universal Enveloping Algebras

Let g be complex semisimple Lie algebra of rank r with root system ∆, Weyl group W , Car-

tan matrix C = (cij) = 2⟨αj, αi⟩/⟨αi, αi⟩ corresponding to the choice of Cartan subalgebra

∗ h and with simple roots Π = {α1, . . . , αr} ⊂ h ; here ⟨−, −⟩ is the invariant bilinear form on the real vector space RΠ normalized by ⟨α, α⟩ = 2 for short roots. Denote by α∨ = 2α/⟨α, α⟩

and sα ∈ W the corresponding coroot and reflection. Let g have the associated triangular de-

composition g = n− ⊕ h ⊕ n+ with corresponding universal enveloping algebra decompostion

U(g) = U(n−) ⊗ U(h) ⊗ U(n+).

Let {ϖ1, . . . , ϖr} be the fundamental weights of g. These are the weights dual to the simple coroots, i.e. those given by the condition

⟨α , α ⟩ ⟨ϖ , α ⟩ = δ d , where d = j j ∈ {1, 2, 3} . (3.1.1) i j i,j j j 2

Note that dicij = djcji, where D = diag(d1, . . . , dn) is the symmetrizer of C. We will denote

the root and weight lattices of g by Q and P and their dominant integral counterparts by ∑r ∑r Q≥0 = Z≥0αi, , and P≥0 = Z≥0ϖi. i=1 i=1 By definition of the fundamental weights. we have that for any λ ∈ Q and root α, the number

⟨λ, α⟩ is an integer. Therefore, by manipulating 3.1.1, we define ⟨⟨−, −⟩⟩ : P × Q −→ Q by

⟨λ, α⟩ ⟨⟨λ, α∨⟩⟩ = 2 (3.1.2) ⟨α, α⟩.

When no confusion exists we will drop the double-angle brackets in favor of single-angle ⟨ ∨⟩ brackets. Note, in particular, that αj, αi is the Cartan matrix entry cij. The simple reflec-

29 tion sα acts on P by

∨ sα(λ) = λ − ⟨λ, α ⟩α ∑ For a parabolic subset I ⊆ Π set Q = Zα . Recall the standard partial order on P I αi∈I i given by

λ1 ≥ λ2 if λ1 = λ2 + γ for some γ ∈ Q≥0 and

λ1 > λ2 if λ1 ≥ λ2 and λ1 ≠ λ2.

∑ ∈ If λ = i∈I niωi P≥0, ni > 0, for all i in I we will say that the support of λ is I. Henceforth, k shall be a field of arbitrary characteristic and q will be a nonzero element

di in k which is not a root of unity. Set qi = q . We will use the following notation in the subsequent sections:

∑ n− −n qn−1 ∞ n qi qi i x − (n)q = exp (x) = [n]qi = − 1 i qi−1 qi n=0 (n)q ! qi qi i

c − −1 [n]qi ! = [1]qi ... [n]qi (n)qi ! = (1)qi ... (n)qi Xi = Xi Xi ( ) [n] ! (n) ! n qi n qi c bb [ k ]q = − = − cX = cX i [k]qi ![n k]qi ! k qi (k)qi !(n k)qi !

Define the quantized universal enveloping algebra Uq(g) to be the associative, unital k-

1 1 algebra with generators E1,...,Er,K1 ,...,Kr ,F1,...,Fr and the following relations:

−1 cij −1 −cij KiEjKi = qi Ej KiFjKi = qi Fj (3.1.3)

−1 c KiKj = KjKi, EiFj − FjEi = δi,jqbi Ki (3.1.4) − 1∑cij [ ] − − 1−cij k 1 cij k F FjF = 0, (i ≠ j) (3.1.5) k qi i i k=0 − 1∑cij [ ] − − 1−cij k 1 cij k E EjE = 0, (i ≠ j) (3.1.6) k qi i i k=0

30 The algebra Uq(g) is graded by the root lattice Q by setting

− 1 deg Fi = αi, deg Ei = αi, deg Ki = 0, (3.1.7)

We label the γ component of Uq(g) by Uq(g)γ. There is a unique (noncommutative, non-

cocommutative) structure on Uq(g) determined by the following comultiplica- tion, antipode, and counit:

⊗ −1 ⊗ − ∆(Fi) = Fi Ki + 1 Fi S(Fi) = FiKi ϵ(Fi) = 0 (3.1.8)

⊗ −1 ∆(Ki) = Ki Ki S(Ki) = Ki ϵ(Ki) = 1 (3.1.9)

⊗ ⊗ − −1 ∆(Ei) = Ei 1 + Ki Ei S(Ei) = Ki Ei ϵ(Ei) = 0 (3.1.10)

For any subset S of Uq(g), let k⟨S⟩ be the subalgebra of Uq(g) generated by S. When confusion should not arise, we will drop the set braces for notational convenience. We set notation for the following important subalgebras to be utilized throughout this paper:

− k⟨{ }r ⟩ 0 k⟨{ }r ⟩ + k⟨{ }r ⟩ Uq = Fi i=1 Uq = Ki i=1 Uq = Ei i=1

≥0 k⟨{ }r ⟩ ≤0 k⟨{ }r ⟩ i k⟨ 1 ⟩ Uq = Ki,Ei i=1 Uq = Fi,Ki i=1 Uq = Fi,Ki ,Ei .

i For any simple root αi, the corresponding algebra Uq is canonically isomorphic to Uqi (sl2).

The isomorphism being Xi 7→ X1. Many arguments regarding Uq(g) are simple extensions of

i those for Uq. For instance,

Lemma 3.1.1. There is a unique algebra automorphism ω, and a unique algebra anti-

2 2 automorphism τ of Uq(g) such that ω = 1 = τ and

−1 ω(Ei) = Fi ω(Fi) = Ei ω(Ki) = Ki

−1 τ(Ei) = Ei τ(Fi) = Fi τ(Ki) = Ki

31 ≥0 ≤0 We call ω the Cartan involution and τ the principal involution. The algebras Uq and Uq

are Hopf subalgebras of Uq(g). There is a unique Hopf pairing

⟨− −⟩ ≥0 op × ≤0 −→ k , :(Uq ) Uq (3.1.11) called the Rosso–Tanisaki form satisfying the following for all i, j ∈ [1, l]:

−⟨αi,αj ⟩ ⟨Ki,Kj⟩ = q ⟨Ki,Fj⟩ = 0

−1 ⟨Ei,Fj⟩ = −δij(qbi) ⟨Ei,Kj⟩ = 0.

Let g have Weyl group W . The Artin braid group of g, in abstract terms, is the group B with generators T1,...,Tr and relations TiTjTiTj ··· = TjTiTjTi ... if and only if we have the braid relation sisjsisj ··· = sjsisjsi ... in W .

(r) Xr i B Let Xi = . The braid group of g acts on Uq(g) by algebra automorphisms [r]qi !

− − − −1 cij TiEi = FiKi,TiFi = Ki Ei,TiKj = KjKi , (3.1.12) − ∑cij − − − k −k ( cij k) (k) ̸ TiEj = ( 1) (qi) Ei EjEi (j = i), (3.1.13) k=0 − ∑cij − − − k k (k) ( cij k) ̸ TiFj = ( 1) (qi) Fi FjFi (j = i). (3.1.14) k=0 Suppose that A is a commutative k-algebra, M is an A-module and λ : A → k is a weight.

In staunch analogy with Lie algebra representation theory, we define the λ-weight spaces of

M by

Mλ = {v ∈ M | a · v = λ(a)v for all a ∈ A}

All modules are left modules unless declared otherwise. Let M be a 1-dimensional repre- sentation of Uq(g). Alternatively, consider the weight λ : Uq(g) → k. The relations in 3.1.3

2 imply that λ(Ei)M = λ(Fi)M = 0 and so λ(Ki )M = M. It is only necessary to consider the case in which Ki acts by 1. See, for instance, [[BG02] Section I.6.12] for a summary.

32 The weight spaces of Uq(g)q as a module over itself are all trivial. Thus we let Uq(g) act on itself by the adjoint representation, or Hopf-algebra conjugation as follows: Let H be a ∑ Hopf algebra with comultiplication ∆(X) = Ai ⊗ Bi. Then adX : H −→ H is defined by ∑ adX (Y ) = AiYS(Bi)

Recalling the Hopf algebra structure of from 3.1.8, we observe the adjoint action of Uq(g) on itself is

−1 adKi (x) = KixKi

⟨λ,αi⟩ To keep track of the weights, we will write λ for the weight satisfying λ(Ki) = q The

(quantized) λ-weight space of the Uq(g)-module Uq(g) is then given by { } − ⟨ ⟩ ∈ | · 1 λ,αi Uq(g)λ = v M Ki v = KixKi = q v for all i in [1, r] . (3.1.15)

In analogy with this, we define the λ-weight space of any Uq(g)-module M by { } ⟨λ,αi⟩ Mλ = v ∈ M | Ki · v = q v for all i in [1, r] . (3.1.16)

Definition 3.1.2. Let M be a Uq(g)-module. We call M integrable (or type 1) if ⊕ M = Mλ. λ∈P

Let M denote the category of finite dimensional, integrable Uq(g)-modules. The first important fact is that M is a semisimple category when q is not a root of unity (see [Jan96,

Theorem 5.17] and the remark on p. 85 of [Jan96]). Furthermore, M is closed under taking

tensor products and duals (defined as left modules using the antipode of Uq(g)). Denote by

V (λ) the irreducible integrable Uq(g)-module of highest weight λ ∈ P≥0. We have the well- known theorem from [[BG02] Section I.6.12], which shows that M is the quantized analogue

of the category of finite dimensional , irreducible U(g)-modules.

33 Theorem 3.1.3. The following hold:

(i) There is a bijection between dominant integral weights and highest weight modules in

M given by

λ. . V (.λ)

(ii) Each V (λ) is the quotient of the Verma module

g Ind (λ) = Uq(g) ⊗ ≥0 k(λ) b Uq

(iii) Each V (λ) has a highest weight vector of weight λ, and the Weyl character formula

gives the weights of V (λ)

(iv) Every module in M is completely reducible.

Now, the braid group B acts on all Uq(g)-modules in a manner that is compatible with

the fact that Uq(g) is a left module over itself; specifically, for all simple roots α, u in Uq(g)

and v in Mλ we have

Tα(x · v) = Tα(x) · Tα(v). (3.1.17)

(see [Jan96, eq. 8.14 (1)]). Let m = ⟨λ, α∨⟩ and explicitly we define the braid automorphisms

′ Tα and Tα by ∑ − b b−ac (a) (b) (c) Tα(v) = ( 1) qα Eα Fα Eα (3.1.18) a,b,c≥0 −a+b−c=m ∑ ′ − b ac−b (a) (b) (c) Tα(v) = ( 1) qα Eα Fα Eα (3.1.19) a,b,c≥0 −a+b−c=m

cf. [Jan96, Section 8.6] and [Lus93, Section 5.2]. Let Tw denote the automorphism Ti1 ...Til for some reduced decomposition of w. When indices are irrelevant, we will label the simple

braid automorphisms by Ti for i ∈ [1, r]. The notation Tw is well-defined according to the following theorem.

34 Theorem 3.1.4 (Lusztig, De Concini–Kac–Procesi). The Ti satisfy the braid group relations,

hence the braid automorphism Tw is independent of the choice of reduced decomposition of w.

In particular we have

Tw(x · v) := (Twx) · (Twv), for all v ∈ V (λ) and λ ∈ P≥0, (3.1.20)

For all w ∈ W , λ ∈ P≥0, and µ ∈ P , the braid group action satisfies

Tw(V (λ)µ) = V (λ)wµ (3.1.21)

From this, the remark [Jan96, Section 5. Remark 1], and the fact that the weights of V (λ)∗

are the negatives of the weights of V (λ) we have

∗ dim V (λ)wλ = dim V (λ)−wλ = 1, (3.1.22)

and w0λ is the unique lowest weight of V (λ). Such weight spaces V (λ)wλ in the Weyl group orbit of a highest weight are called extremal weight spaces. In subsequent sections we will make use of

Lemma 3.1.5. Let vλ be a highest weight vector of highest weight λ = nϖi for an irreducible,

i ∈ Z ≤ finite dimensional, integrable Uq-module. For all m, n ≥0 such that m n we have

− n (n) −1 (n) (i) Ti(vλ) = ( qi) Fi vλ, and Ti (vλ) = Fi vλ,

m m [m]i![n]i! (ii) Ei Fi vλ = vλ [n − m]i!

35 Proof. By [Jan96, eqs. 8.6 (6), (7)] we have

−1 − n ′ Ti (vλ) = ( qi) Ti (v) ∑ − n − b ac−b (a) (b) (c) = ( qi) ( 1) qi Ei Fi Ei vλ a,b,c≥0 −a+b−c=m − n − −n (n) = ( qi) ( qi) Fi vλ

(n) = Fi vλ

− n (n) The proof of Ti(vλ) = ( qi) Fi vλ is equally straightforward and omitted . Now, observe

∏m m m j−m − −(j−m) c Ei Fi vλ = [m]qi ! (q q )Kivλ j=1 ∏m − = [m]qi ! [n m + j]i vλ j=1 − = [m]qi ![n m + 1]i ... [n]ivλ

[m]qi ![n]qi ! = − vλ [n m]qi !

The first equality follows from [Jan96, Lemma 1.7] and the second from [Jan96, eq. 5.12 (7)].

The third and forth follow from common sense.

We will also use the following fact regarding the previously defined 3.1.1 principal involu- tion τ: For all x ∈ Uq(g) and w ∈ W , τ satisfies

−1 τ(Twx) = Tw−1 (τ(x)). (3.1.23)

−1 In particular, we have τ(Tix) = Ti (τ(x)) see [Jan96, eq. 8.18(6)].

3.2 Quantized Coordinate Rings

We construct Rq[G] the (single parameter) q-quantized coordinate ring of a connected, simply connected, complex semisimple algebraic group G with Lie algebra g. Let A be a

36 k-algebra. We define the finite or restricted dual A◦ to be the algebra with underlying set

{f ∈ Hom(A, k) | f(I) = 0 on some ideal of A satisfying dimkA/I < ∞} (3.2.1)

Intuitively, these are the functionals which vanish on almost all of A (in an algebraic sense). It is well-known that A◦ is a with comultiplication and counit induced by Hom(A, −).

We pause here to draw an analogy with the universal enveloping algebra . For the moment allow G to be any algebraic group over k with Lie algebra g. Another interpretation of U(g) is the algebra of k-valued algebraic functions on the coordinate ring O(G) that die on a formal neighborhood of the identity of G. Explicitly, let me be the ideal of functions in O(G) which vanish at the identity of G. It is well known that

∼ { ∈ O ∗ | n } U(g) = f (G) f(me ) = 0 for some n > 0 .

In general U(g)◦ is not isomorphic to O(G), but we have the following classical result:

Theorem 3.2.1 (Cartier). For a semisimple, connected and simply connected algebraic group

G over an algebraically closed field of characteristic 0

∼ ◦ O(G) = U(g)

We wish to mimic the behavior Theorem 3.2.1 in the non-classical setting. To this end let

H be a Hopf algebra, and suppose M is an H-module. For any f ∈ Hom(M, k) and v ∈ M,

M ∈ k ∗ we define the coordinate function cf,v Hom(H, ) = H determined by pairing f against xv for any x ∈ H. Explicitly,

M x . . .cf,v(x) = f(xv). (3.2.2)

M ◦ Lemma 3.2.2. If M is a finite dimensional H-module, then cf,v(x) is in H .

M Proof. Note that if s is the annihilator of M, then cf,v(s) = 0. Thus, we may take I = AnnM in the notation of 3.2.1.

37 Recall M —the category of finite dimensional, integrable Uq(g)-modules. Let M+ be the full subcategory of M consisting of modules M satisfying ⊕ M = V (λ) (3.2.3)

λ∈P≥0

◦ We define Rq[G] to be the Hopf subalgebra of Uq(g) generated by coordinate functions of the modules in M+. Equivalently, Rq[G] is generated by the coordinate functions of the

∈ M ∈ λ highest weight modules V (λ) for λ P≥0. We abbreviate these functions to cf,v and summarize:

λ ∈ ∈ ∗ ∈ cf,v(x) = f(xv) v V (λ) f V (λ) x Uq(g)

For any bialgebra H we may endow all of H∗ with a ring structure by setting ∑ ∑ fg(x) = f(x1) ⊗ g(x1), where ∆(x) = x1 ⊗ x2 (3.2.4)

Here we are using Sweedler’s notation for ∆(x). Thus, Rq[G] inherits this standard ring structure. For the sake of clarity we investigate multiplication in Rq[G]. For all x we have the string of equalities ∑ ∑ cf,ucg,v(x) = cf,u(x1)cg,v(x2) = f(x1u)g(x2v) ∑ = (f ⊗ g)( x1u ⊗ x2v) = f ⊗ g(x(u ⊗ v))

= cf⊗g,u⊗v(x)

Thus, we have shown cf,ucg,v = cf⊗g,u⊗v(x).

It will be useful to keep in mind an alternative formulation of Rq[G] that is independent from the coordinate functions. If H is a bialgebra, then H∗ has a natural H–H-bimodule structure given by

(a.f)(x) = f(xa) and (f.a)(x) = f(ax),

38 where a, x ∈ H and f ∈ H∗. A straightforward check shows H◦ is a sub-bimodule of H∗.

◦ Thus, Uq(g) is naturally a Uq(g)–Uq(g)-bimodule with structure arising from the bimodule

◦ structure on Uq(g). We can express the left and right actions of x ∈ Uq(g), on c ∈ Uq(g) ∑ given by the following: If ∆(f) = f1 ⊗ f2, then set ∑ f.a(−) = f2(a)f1(−) and (3.2.5) ∑ a.f(−) = f1(a)f2(−) (3.2.6)

Define the double weight space (compatible with the construction in 3.1.15) by { } ◦ ◦ ⟨ ⟩ − ⟨ ⟩ ∈ | · λ1,αi · 1 λ2,αi Uq(g)λ2λ1 = v Uq(g) Ki v = q v and v Ki = q for all i in [1, r] (3.2.7)

We then have ⊕ ◦ Rq[G] = Uq(g)λ2λ1 λ1,λ2∈P

We remark that the notation Rq[G] is common although it does not include the field k on

which the construction of Uq(g) depends. Thus, G is more of a suggestive symbol. 3.3 Weyl group combinatorics

Let W be the Weyl group of a fixed rank r Lie algebra g, and let S = {s1, . . . , sr} be a

set of generators for W corresponding to a choice of simple roots Π = {α1, . . . , αr} for g,

that is, sj is the simple reflection sαj . Let [m, n] denote the set of integers from m to n. For simplicity of notation, we define the following: If i = i1 . . . il is an expression or string of numbers in [1, r], then the corresponding Weyl group element si in the symbols of S is given by mapping a subword to its index set.

si := si1 . . . sil

By construction there is a natural bijection D : Sw −→ Pl between subwords of si and the 2l subsets of [1, l] given by

s . . . s 7−→ {j , . . . , j } (3.3.1) ij1 ijm 1 m

39 We suggestively denote the empty subword of si by 1. Let us pause to stress that we consider

elements in Sw to merely be subwords and not Weyl group elements; therefore, for example, ̸ S ≤ ··· ≤ sij sij = 1 as elements of w . Observe 1 j1 < < jm l by construction. With this

bijection in mind, we set the following notation: For each expression i = i1 . . . in and each subset D ⊂[1, l] set    sj if j ∈ D sD = j  1 if j∈ / D

D D−1 we define si := (D). Explicitly, we have

D D D si = s1 . . . sn . (3.3.2)

D P ∼ ′ D D′ Let l be the quotient of l defined by D D if si and si are equal as Weyl group

elements. Elements in Dl are called diagrams. Note that in general D does not descend to

a bijection Sw onto Dl. In the spirit of a running example, observe that if i = 2132, then

|Pl| = 16, while |Dl| = 14. To see this we simply observe

{1} ∼ {4} and {1, 4} ∼ ∅.

We will be interested in the subset of Sw that D maps bijectively onto Dl. To this end we define the standard length function on W ; it is a map

ℓ : W −→ Z≥0

where ℓ(w) is defined to be the smallest number l so that w is a product of l simple reflections.

Example 3.3.1. Let i = 12121 so that si determines an element in Σ3, the symmetric group

on three symbols. We have ℓ(si) = 1 since s12121 = s12212 = s2.

By definition it is clear that

(i) ℓ(1) = 0

40 (ii) ℓ(w) = ℓ(w−1)

(iii) ℓ(ww′) ≤ ℓ(w) + ℓ(w′)

If a length l word i = i1 . . . il in the elements of [1, r] has the property ℓ(si) = l, then i is

called a reduced expression for the Weyl group element represented by si. When i is a reduced expression for w, we write w to denote the representative si. By finiteness W contains an element of maximal length. Now, if w ∈ W is any element and si ∈ S a simple reflection, then

ℓ(siw) = ℓ(w)  1

Theorem 3.3.2. There is a unique maximal length element w0 of W called the longest word. It is characterized by the property

ℓ(siw0) < ℓ(w0) for all simple reflections si

Proof. Existence follows immediately from finiteness of W . Such a word can be constructed

from any maximally long sequence i satisfying

≤ ≤ · · · ≤ ℓ(si1 ) ℓ(si1i2 ) ℓ(si)

−1 Let w1 be any other maximal length word. Uniqueness follows from showing ℓ(w0w1 ) = 0.

−1 If not then w0w1 = sj for some reduced expression. We proceed by induction on the length

of j. Note that the length of j cannot be 1, otherwise ℓ(w0) = ℓ(w1) − 1. Similarly, j cannot

be length two. because w0 = si1 si2 w1

Henceforth, let the longest word have length N. Multiple reduced expressions for the

longest word exist for Weyl groups containing at least two simple generators. In general

many reduced expressions may exist for a Weyl group element. With this in mind, we define

41 a partial order on W . Let i be a reduced expression for w. The Bruhat order on W is defined by

w′ ≤ w if some reduced word for w′ is a subword of i.

The intial Bruhat order interval W ≤w is defined by

W ≤w = {v ∈ W | v ≤ w}

≤w D By construction exp : Dl −→ W (determined by D 7→ w ) is a bijection. For m and n in [0, l], we define the truncations of w by   ≤ sim sim+1 . . . sim−1 sin if m n (3.3.3) w[m,n] =  sim sim−1 . . . sin+1 sin if m > n (3.3.4)

We set w[j,j] = wj for notational simplicity, and we adopt the convention w0 := 1. Observe the

−1 simple (but useful) facts w[1,l] = w and w[m,n] = w[n,m]. Furthermore, observe that truncation

D commutes with diagram exponentiation so the notation w[1,j], for instance, is unambiguous. Following [MR04] we call a subexpression y of w right positive if its diagram D = D(y) has the property that D D ∈ − w[1,j]sij+1 > w[1,j] for all j [1, l 1]. and left positive if D D ∈ − sij w[j+1,l] > w[j+1,l] for all j [1, l 1]. (3.3.5)

Let Lw denote the collection of left positive subexpressions of the reduced expression w corresponding to the reduced word i.

Example 3.3.3. Let i = 2132 so that w = s2s1s3s2. Then every subset of {1, 2, 3, 4} except for {1, 4} and {4} corresponds to left positive subexpressions. Both sets fail immediately

since

{1,4} {1,4} 1 = s2(s1s3s2) ≯ (s1s3s2) = s2

42 Analogously, we see that that every subexpression of w is right positive except (again) s2s2

and s2, this time though with index sets {1, 4} and {1}.

≤w The following lemma is the first step to establishing a bijection W −→ Lw .

Lemma 3.3.4 (Marsh–Rietsch). For any reduced expression i for w and any element y ∈

W ≤w, there is a unique right positive subexpression for y.

≤w Corollary 3.3.5. For any y ∈ W , there is a unique left positive subword y ∈ Lw whose diagram D = D(y) satisfies wD = y.

−1 Proof. The inversion map i = i1 . . . il 7→ il . . . i1 = i gives rise to a bijection between the left positive subwords of w and the right positive subwords of w−1. The claim follows because

− the induced (honest inversion) map W ≤w −→ W ≤w 1 defined by y 7→ y−1 is a bijection.

The bijection L arising from corollary is represented by the diagram

≤w L D exp ≤w W . L.w D.l W . .

y. y. D. wD .= y

We will denote the composition D ◦ L by D+; this is the left positive diagram bijection to be heavily used in subsequent sections. We now summarize the results and observations of this section. For any reduced word i we have the commutative diagram

D ∼ Sw. P.l D.l D . exp

L ≤w .w L. W . . 3.4 Explicit construction

In this section we outline the original construction of the upper and lower quantum Schu-

bert cell algebras U [w] due to De Concini–Kac–Procesi[DCKP95] and Lusztig [Lus93, Sec-

tion 40.2]. We also gather relevant facts regarding these algebras. The naming suggests that,

43 from the quantized ring of functions vantage-point, that these algebras are deformations of

the coordinate ring of Schubert cells BwB/B. This is compatible with the underlying theme

quantum groups are quantized coordinate rings of algebraic groups.

For every w ∈ W set

∆w = ∆+ ∩ w∆−.

−1 This is the set of positive roots α such that w α < 0; in particular ∆w0 = ∆+, where w0

denotes the longest word. For each reduced word i = i1 . . . il for w, we obtain an ordered list

of distinct positive roots β1, . . . , βl defined by

βj = w[1,j−1]αij (3.4.1)

It is well known that ∆w = {β1, . . . , βl}. Recall the Lusztig braid group action on Uq(g) 3.1.12. In complete analogy with the previous construction of positive roots, we define an

ordered list of positive (negative) Lusztig root vectors. Following Lusztig [Lus93, Section

39.3] we define the positive root vectors Eβ1 ,...,Eβl and the negative ones Fβ1 ,...,Fβl by

E = T E and F = T E . (3.4.2) βj w[1,j−1] ij βj w[1,j−1] ij

l For every m = (m1, . . . , ml) ∈ (Z≥0) and i, j ∈ [1, l], define the standard monomials  m − m Xmi Xmi+1 ...X j 1 X j if i ≤ j βi βi+1 βj βj m[i,j] X =  Xmi Xmi−1 ...Xmj+1 Xmj if i > j βi βi+1 βj βj

Let us adopt the conventions Xm = Xm[1,l] and Xm(i,j) = Xm[i+1,j−1] or Xm[i−1,j+1] (according

to whether i < j).

Theorem 3.4.1 (Lusztig, De Concini–Kac–Procesi). Let ℓ(w0) = N and suppose w has

reduced word i = (α1, . . . , αl), then

⊂ + (i) We have the containment Eβ1 ,...,EβN Uq

44 { } + (ii) If βj = αi, then Eβj = Ei. Thus, Eβ1 ,...,EβN contains all the generators of Uq .

+ k⟨ ⟩ + (iii) Define U [w] = Eβ1 ,...,Eβl , the subalgebra of Uq generated by Eβ1 ,...,Eβl . The collection of monomials {Em} forms a vector-space basis for U +[w].

+ + (iv) U [w0] = Uq .

− − We have analogous results for U [w]—the subalgebra of Uq generated by Fβ1 ,...,Fβl . The basis {Fm} is called a Poincar´e–Birkhoff–Witt basis because specializing to q = 1, we

+ obtain the usual PBW basis for U(n ∩ wn−). The following lemma [DCKP95, Prop. 2.2]

and [Lus93, Prop. 40.2.1] shows that the notation U +[w] is appropriate.

Proposition 3.4.2 (De Concini–Kac–Procesi, Lusztig). The algebras U [w] do not depend on the choice of reduced word i for w.

The Levendorski˘i–Soibelman straightening law will play a major role in our study of U −[w].

Proposition 3.4.3 (Levendorski˘i–Soibelman ). For i < j there are structure constants

cm ∈ k such that [j,i] ∑ −⟨ ⟩ − βi,βj m(j,i) Fβj Fβi q Fβi Fβj = cm[j,i] F . (3.4.3) m[j,i] The support of w ∈ W is defined by

S(w) = {i ∈ [1, r] | si ≤ w}. (3.4.4)

Its complement is given by

Π\S(w) = {i ∈ [1, r] | wϖi = ϖi}, (3.4.5)

see [Yaka, Lemma 3.2 and equation (3.2)]. Now observe that the Q-grading on Uq(g) 3.1.7

  induces a Q-grading on U [w]. We will label the γ graded component by U [w]γ; we have the following easy fact

 Z{γ ∈ Q | U [w]γ ≠ 0} = QS(w), (3.4.6)

45 see e.g. [Yaka, equation (2.44) and Lemma 3.2 (ii)].

Recall the unique ω involution 3.1.1 of Uq(g). It satisfies

⟨α∨,γ⟩ −⟨α,γ⟩ ω(Tα(u)) = (−1) q Tα(ω(u)), for all γ ∈ Q and u ∈ Uq(g)γ,

see [Jan96, equation 8.14(9)]. In other words, if ρ is the sum of all fundamental weights of g

∨ ∨ ⟨sα(γ)−γ,ρ ⟩ −⟨sα(γ)−γ,ρ⟩ and ρ is the sum of all fundamental coweights of g, then ω(Tα(u)) = (−1) q Tα(ω(u))

for u ∈ Uq(g)γ. Thus

⟨y(γ)−γ,ρ∨⟩ −⟨y(γ)−γ,ρ⟩ ω(Ty(u)) = (−1) q Ty(ω(u)), for all y ∈ W, γ ∈ Q, u ∈ Uq(g)γ,

see [Jan96, equation 8.18(5)] for an equivalent formulation of this fact. In particular, the

restrictions of ω induce the isomorphisms

− ⟨ − ∨⟩ −⟨ − ⟩ + → − βj αj ,ρ βj αj ,ρ ∈ ω : U [w] U [w], ω(Eβj ) = ( 1) q Fβj for all j [1, l]. (3.4.7)

To each γ ∈ Q associate a character as follows: Let t ∈ H be given by (t1, . . . , tr), and

× define the multiplicative character Xγ ∈ Hom(H, k ) by

⟨ ⟩ γ,ϖ1 ⟨γ,ϖr⟩ Xγ(t) = t1 . . . tr (3.4.8)

The torus H acts rationally on Uq(g) by characters. Specifically, if ∈ Uq(g)γ, then we set

⟨ ⟩ ⟨ ⟩ · γ,ϖ1 γ,ϖr t x = Xγ(t)x = t1 . . . tr x. (3.4.9)

This torus-action preserves the subalgebras U [w]. We will denote by H-SpecU −[w] the set

of H-prime ideals of U −[w]. We can endow this H-spectrum with the Zariski topology, but

this topology will play no role in this paper.

Fix a length l reduced word i for w and recall the positive roots βj from equation (3.4.1).

Equation (3.4.6) implies that for all j ∈ [1, l] there exists a unique tj = (tj,1, . . . , tj,r) ∈ H ≤ ∈ \S such that for all k j and i [1, r] (w[1,j]), respectively, we have

⟨ ⟩ ⟨ ⟩ βk,ϖ1 βk,ϖr ⟨βk,βj ⟩ Xβk (tj) = tj,1 . . . tj,r = q , and tj,i = 1. (3.4.10)

46 See, for example, [BG02, Proposition I.6.10] for details. The following lemma is a direct con-

sequence of Theorem 3.4.1 (iii), equation (3.4.10), and the Levendorski˘i–Soibelman straight-

ening law in Proposition 3.4.3.

Lemma 3.4.4. For all fields k with q ∈ k× not a root of unity, Weyl group elements w,

reduced words i = (i1, . . . , il) for w, and j ∈ [1, l] we have:

− − (i) The subalgebra of U [w] generated by Fβ1 ,...,Fβj is equal to U [w[1,j]].

− ∼ − (ii) There is an algebra isomorphism U [w[1,j]] = U [w[1,j−1]][xj, σj, δj] given by the identity − 7→ − − ∼ k on U [w[1,j−1]] and Fβj xj. Furthermore, U [w0] = U [1] = , σ1 = id, and δ1 = 0.

· (iii) The eigenvalues tj Fβj = qαj Fβj are not roots of unity.

· − In Lemma 3.4.4 (ii) we have σj = (tj ) as automorphisms of U [w[1,j−1]], and δj is a locally nilpotent σ -derivation of U −[w ] satisfying σ δ = q−2δ σ . This σ -derivation δ j [1,j−1] j j ij j j j j is explicitly given by

⟨ ⟩ − − βj ,γ ∈ δj(x) = Fβj x q xFβj , for x (U [w[1,j−1]])γ (3.4.11)

and is computed using the Levendorski˘i–Soibelman straightening law. The isomorphisms in

Lemma 3.4.4 (ii) give rise to the Ore extension presentations

− − ≤ ≤ U [w[1,j]] = U [w[1,j−1]][Fβj , σj, δj], for all 1 j l.

Inductively, we obtain a presentation for U −[w] as an iterated Ore extension for any reduced word i for w given by

− k U [w] = [Fβ1 ][Fβ2 ; σ2, δ2] ... [Fβl ; σl, δl].

47 Chapter 4 Ring vs representation theoretic approaches to SpecU −[w]

4.1 Cauchon’s effacement des d´erivations

We outline Cauchon’s ring theoretic approach to the study of SpecU −[w] via the method

of deleting derivations. We follow [Cau03a,MC10] and the review in [BL10, Section 2].

Fix an iterated Ore extension

A = k[x1][x2; σ2, δ2] ... [xl; σl, δl], (4.1.1)

For all j ∈ [2, l], σj is an automorphism and δj is a (left) σj-skew derivation of

A[1,j−1] = k[x1][x2; σ2, δ2] ... [xj−1; σj−1, δj−1].

Definition 4.1.1. An iterated Ore extension A as in equation (4.1.1) is called a Cauchon–

Goodearl–Letzter (CGL) extension if it is equipped with an action of an algebraic torus

H = (k∗)l by algebra automorphisms satisfying the following conditions for all 1 ≤ i < j ≤ l:

(i) σj(xi) = qj,ixi for some qj,i ∈ k

(ii) δj is a locally nilpotent σj-skew derivation of A[1,j−1]

(iii) The elements x1, . . . , xl are H-eigenvectors and the following set is infinite:

{p ∈ k | there is a t in H satisfies t · x1 = px1} .

∗ (iv) There exists tj ∈ H such that tj · xj = qjxj for some qj ∈ k , which is not a root of

unity, and tj · xi = qj,ixi, for all i ∈ [1, j − 1]

A length l CGL extension is called torsion-free if the subgroup of k∗ generated by all qj,i, 1 ≤ i < j ≤ l is torsion-free. Condition iv is merely the statement that σj = (tj·) as

automorphism of A[1,j−1].

48 Lemma 4.1.2. For all CGL extensions A = R[x; σ, δ], we have σδ = qδσ.

Proof. For all r ∈ R we have

δ(r) = xr − σ(r)x. (4.1.2)

This follows directly from the multiplication in A. Upon applying σ, we obtain

σ(δ(r)) = σ(x)σ(r) − σ2(r)σ(x)

= qxσ(r) − qσ2(r)x

On the other hand, equation (4.1.2) is valid in particular for r = σ(r) yielding

δ(σ(r)) = xσ(r) − σ2(r)x.

Thus, σδ(r) = qδσ(r).

By applying the lemma to any length l Ore extension, we obtain σjδj = qjδjσj for all j ∈ [2, l]. Given a CGL extension A as in (4.1.1), for j = l + 1, l, . . . , 2, Cauchon iteratively constructed in [Cau03a] l-tuples of nonzero elements

(j) (j) (x1 , . . . , xl )

and families of subalgebras of Frac(A) given by

(j) k⟨ (j) (j)⟩ A = x1 , . . . , xl .

We begin by setting

(l+1) (l+1) (l+1) (x1 , . . . , xl ) = (x1, . . . , xl) and A = A.

(j) (j) (j+1) (j+1) For j = l, . . . , 2, the l-tuple (x1 , . . . , xl ) is determined from (x1 , . . . , xl ) by  (j+1) ≥ xi if i j (4.1.3) (j) ∞ [ ( )]( ) x = ∑ (1 − q )−m −m i  j δmσ−m x(j+1) x(j+1) if i < j. (4.1.4) (m) ! j j i j m=0 j

49 For j ∈ [2, l + 1], Cauchon constructed an algebra isomorphism

∼ (j) = A . . .k[y1] ... [yj−1; σj−1, δj−1][yj; τj] ... [yl; τl], (4.1.5)

where τk denotes the automorphism of k[y1] ... [yj−1; σj−1, δj−1][yj; τj] ... [yk−1; τk−1] such that

τk(yi) = qk,iyi for all i ∈ [1, k − 1]. This variable substitution isomorphism is given by (j) ∈ xi . . .yi, i = 1, . . . , l. For each j [2, l], define {( ) } m (j+1) ∈ Z Sj = xj m ≥0 .

(j) (j+1) (j) −1 (j+1) −1 Then Sj is an Ore subset of A and A . Moreover, we have A [Sj ] = A [Sj ] by [[Cau03a] Th´eor`em3.2.1. (1)]. ∈ −1 ≤ ≤ Set qi,i = 1 for i [1, l] and qi,j = qj,i for 1 i < j l. The multiparameter quantum

l affine space algebra Rq[A ] associated to the multiplicatiively antisymmetric l × l matrix

q = (qi,j) is the k-algebra with generators X1,...,Xl and relations

XiXj = qi,jXjXi, for all i, j ∈ [1, l].

We will call the algebra A = A(2) obtained at the end of the Cauchon deleting derivation procedure the Cauchon space of A; the final l-tuple of generators of A will be denoted by

(2) (2) (x1,..., xl) = (x1 , . . . , xl ). For j = 2 equation (4.1.5) gives an isomorphism

∼ = n A . . .Rq[A ] determined by xi . . .Xi. (4.1.6)

We now proceed by describing the induced set-theoretic embeddings

(j+1) (j) φj : SpecA . . . SpecA (4.1.7)

for j ∈ [2, l]. By Goodearl and Letzter [GL00, Proposition 4.2] all H-prime ideals of a CGL

extension are completely prime (recall equation (4.1.5)), and by another result of Good-

earl and Letzter [GL94, Theorem 2.3] all prime ideals of a torsion-free CGL extension are

50 completely prime. Thus, the above mentioned condition is satisfied for all torsion-free CGL

extensions A because of equation (4.1.5). Now, suppose J is in H-SpecA(j+1) or SpecA(j+1).

As an immediate corollary of the preceeding observation, we have

(j+1) ∈ ⇒ ∩ ∅ xj / J J Sj+1 = .

(j) (j+1) (j+1) Let gj : A . . .A /(xj ) be the homomorphism

(j) (j+1) (j+1) ∈ gj(xi ) = xi + (xj ) for i [1, l]. (4.1.8)

The induced spectrum map in (4.1.7) is given by  −1 ∩ (j) (j+1) ∈ JSj A if xj / J (4.1.9) φj(J) = ( )  −1 (j+1) (j+1) ∈ gj J/(xj ) if xj J (4.1.10) This constructions applies, in particular, when A = U −[w] as these algebras are torsion-free

CGL extensions when q ∈ k∗ is not a root of unity by 3.4.4. We summarize this conditional

set theoretic embedding following diagram:

(j+1) (j+1) (j+1) (j+1) −1 SpecA . /(xj ) . SpecA. . SpecA . Sj g−1 . φj

(j) (j) −1 . . Spec.A . SpecA. Sj

Setting φ = φ2 . . . φl we have the set inclusion

φ φ − φ SpecA. (l+1) l Spec.A(l) . l 1 .... 2 Spec.A(2)

from SpecA to SpecA. This map restricts to an inclusion

φ : H-Spec. A . H-Spec. A, (4.1.11)

which we also label φ. Now, A is a quantum affine space by (4.1.6). Hence, the H-prime

ideals of A are in bijection with the subsets of [1, l], that is, they are of the form

KD = A⟨xi | i ∈ D⟩ for D ⊆ [1, l].

51 The Cauchon diagram of J ∈ H-SpecA is the unique set D ⊆ [1, l] such that φ(J) = KD. We will denote the Cauchon diagram of J by C(J). If D ⊆ [1, l] is the Cauchon diagram of

a H-invariant prime ideal of A, then this prime ideal will be denoted by

−1 JD = φ (KD). (4.1.12)

To define the leading terms of an ideal in A, we set

A[1,l−1] = k⟨x1, . . . , xl−1⟩ = k[x1][x2; σ2, δ2] ... [xl−1; σl−1, δl−1] and (4.1.13)

(l) k⟨ (l) (l) ⟩ A[1,l−1] = x1 , . . . , xl−1 so that we have the equalities

(l) (l) (l) −1 (l) 1 A = A[1,l−1][xl; σl, δl],A = A[1,l−1][xl; τl], and A Sl = A[1,l−1][xl ; τl].

(l) H (l) It is clear that A[1,l−1] and A[1,l−1] are -stable subalgebras of A and A , respectively. Furthermore, the deleting derivations map establishes an H-equivariant algebra isomorphism

∼ . = (l) θ : A[1. ,l−1] A[1,l.−1] (4.1.14)

determined by ∞ ∑ (1 − q )−m x. x(l) = . l . [δmσ−m(x )]x−m. (4.1.15) i i (m) ! l l i l m=0 ql

for i ∈ [1, l − 1]. Suppose J an ideal of A. Each nonzero element of J can be written in the form

m m−1 ··· j = amxl + am−1xl + + a1xl + a0,

e where am ≠ 0 and each ai is in A[1,l−1]. We let J denote the collection of all such am as j ranges over J. We call this the leading part of J. We will make use of the equality of the left

52 and right leading parts of J, that is,

{ } e ∈ | ∈ ∈ Z − m ∈ m−1 J = a A[1,l−1] there exists b J and m ≥0 such that a bxl A[1,l−1]xl + ... + A[1,l−1] (4.1.16) { } ∈ | ∈ ∈ Z − m ∈ m−1 = a A[1,l−1] there exists b J and m ≥0 such that a xl b xl A[1,l−1] + ... + A[1,l−1] .

This follows because σl is locally finite. Recall a regular element in a ring R is an element that is neither a right nor a left zero

divisor. The proof of the following lemma is analogous to [KL00, Lemma 4.7] and is left to

the reader.

Lemma 4.1.3. Let x be a regular element of the k-algebra A for which there exist two k- linear maps σ, δ : A . . .A such that σ is locally finite, δ is locally nilpotent, σδ = qδσ for

some q ∈ k∗, and

xa = σ(a)x + δ(a), for all a ∈ A.

Then the set Ω = {1, x, x2,...} is an Ore subset of A and

GKdimA[Ω−1] = GKdimA.

Suppose J is an H-prime ideal of a CGL extension such as in equation (4.1.1), there are two

possibilities—either xl is in J or it is not. This simple observation paired with the following two important proposition will afford us methods for recursive computation.

Proposition 4.1.4 (G., Yakimov). Let A be a length l CGL extension, and assume J is in

H-SpecA. If xl ∈/ J, then ⊕ ⊕ −1 e m e m (i) JSl = θ(J)xl and φl(J) = θ(J)xl m∈Z m∈Z≥0

(ii) C(J) = C(Je), and

53 e (iii) GKdim(A/J) = GKdim(A[1,l−1]/J) + 1.

H −1 (l) 1 Proof. Part i: By [LLR06, Lemma 2.2] every -invariant ideal L of ASl = A[1,l−1][xl ; τl] has the form ⊕ m (l) L = L0xl for some ideal L0 of A[1,l−1]. (4.1.17) ∈Z ∑ m m H Indeed, if a = m amxl is in the -invariant ideal L, then ∑ k · −k k km m ∈ Z tl (xl axl ) = ql amxl is in L for all k ≥0, m ∈ H m ∈ ∈ Z where tl is the element from Defeinition 4.1.1 iv. Thus, amxl L, for all m , which proves equation (4.1.17). −1 ∈ We apply this to the ideal L = JSl . Equation (4.1.15) implies that for all a A[1,l−1] and m ∈ Z m∑−1 m m k θ(a)xl = axl + bkxl k=m−n ≥ ∈ −1 ⊂ −1 for some n 0, and bk A[1,l−1]. Now, every nonzero element of JSl A[1,l−1]Sl has the form m∑−1 m k ∈ e\{ } ∈ j = axl + akxl for some a J 0 , and ak A[1,l−1]. k=−n It should also have the form

m∑−1 m ′ k ∈ e\{ } ′ ∈ (l) j = θ(a)xl + akxl for some a J 0 , and ak A[1,l−1]. k=m−n

Now the two equalities in i follow from equation (4.1.17). Claim ii that C(J) equals C(Je) is

immediate from the definition of Cauchon diagrams, and claim iii easily follows from Lemma

4.1.3 and the isomorphism

−1 ∼ e  (A/J)[Sl ] = θ(A[1,l−1]/J)[xl , τl].

54 We will also make use of the following proposition, which is complementary to Proposition

4.1.4:

Proposition 4.1.5 (G., Yakimov). Let A be a length l CGL extension, and assume J is in

H-SpecA. If xl ∈ J, then

(l) (i) φl(J) = θ(J ∩ A[1,l−1]) + A xl,

(ii) C(J) = C(J ∩ A[1,l−1]) ⊔ {l}, and

(iii) there are H-equivariant algebra isomorphisms

∼ (l) ∼ ∩ ∼ (l) ∩ (l) A/J = A /φl(J) = A[1,l−1]/(J A[1,l−1]) = A[1,l−1]/(φl(J) A[1,l−1]).

In particular, GKdim(A/J) = GKdim(A[1,l−1]/J ∩ A[1,l−1]).

Proof. Statements i and ii follow from the definition of φl. The latter also implies that

(l) (l+1) (l+1) gl : A . . .A /(xl ) = A/(xl)

(l) ∼ ∼ induces the H-equivariant algebra isomorphism A /φl(J) =(A/(xl)) /(J/(xl)) = A/J. Since xl ∈ J and xl ∈ φl(J) the inclusions

. (l) . (l) A[1,l.−1] A. and A[1,l.−1] A.

induce the H-equivariant algebra isomorphisms

∼ ∼ ∩ . = (l) ∩ (l) . = (l) A[1,l−1]/(J. A[1,l−1]) A/J. and A[1,l−1]/(φl(J. ) A[1,l−1]) A /φ. l(J)

H (l) ∼ The claim follows by the -equivariant isomorphism A[1,l−1] = A[1,l−1] from 4.1.14.

All quantum Schubert cell algebras U −[w] over arbitrary fields k such that q ∈ k∗ is not a root of unity are torsion-free CGL extensions by equation 3.4.4. Moreover, each reduced

word i for w gives rise to a presentation (5.1.1) of U −[w] as a CGL extension. We conclude

this section by recalling the following classification of H-SpecU −[w].

55 Theorem 4.1.6. (Cauchon–M´eriaux, [MC10]) For any simple Lie algebra g, Weyl group element w, reduced word i for w, and any field k such that q ∈ k∗ not a root of unity, let

U −[w] have the presentation given in equation (5.1.1). In this presentation of U −[w] as a

− torsion-free CGL extension, the H-prime ideals of U [w] are the ideals JD+(y) for the elements y ∈ W ≤w (recall equation (4.1.12)), where D+(y) ⊆ [1, l] is the index set of the left positive

subword of i whose total product is y, cf. Section 3.3.

This theorem gives a bijection between Cauchon diagrams of the H-invariant prime ideals

of U −[w] and the index sets of all left positive subwords of i. We note that in [MC10] Theorem

4.1.6 was formulated for the upper quantum Schubert cells U +[w]. However, these formula-

tions are equivalent by the Cartan involution from (3.4.7). We give a second, independent

proof of this theorem in Section 5.2. 4.2 The prime spectrum of U −[w] via Demazure modules

We proceed with the realization of the algebras U −[w] in terms of quantum function algebras and the description of the spectra of U −[w] via Demazure modules from [Yak10].

Recall that, for λ ∈ P≥0 and w ∈ W , the extremal weight spaces V (λ)wλ are all one dimensional and that the braid group permutes the weight spaces of the V (λ) (see 3.1.15

w −1 λ ∈ ∗ and 3.1.21). We then have vλ = Tw−1 vλ is in V (λ)wλ, and there is a unique fw (V (λ) )−wλ such that ⟨ λ w⟩ fw, vλ = 1. (4.2.1)

Also, recall the previously defined coodinate functions 3.2. For each dominant integral weight

λ ∈ ◦ λ, fix a highest weight vector vλ of V (λ)λ, and let cf Uq(g) be given by

λ cf (x) = f(xvλ)

Note that cλ = cλ . Now we define the subalgebra R of R [G] by f f,vλ q { } λ | ∈ ∈ ∗ R = Span cf λ P≥0 and f V (λ)

56 ∈ ∈ λ ∈ λ ∈ For y, w W and λ P≥0, define the elements ew,y Rq[G] and ew R by

λ λ ew,y(x) = fw(xTyvλ) (4.2.2)

λ λ ew(x) = fw(xvλ). (4.2.3)

λ λ ϖ Note that ew = ew,1. When ϖ is a fundamental weight, the element ew is called a quantum minor, cf. [BZ05].

Proposition 4.2.1. For all λ1, λ2 ∈ P≥0 and w ∈ W, we have

λ1 λ2 λ1+λ2 λ2 λ1 ew ew = ew = ew ew . (4.2.4)

Proof. Proved analogously to [Yaka, eq. (2.18)] using the second equality in (i).

{ λ | ∈ } ⊂ Joseph proved the multiplicative subsets Ew = ew λ P≥0 R are Ore sets, see [Jos95, Lemma 9.1.10]. We remark Joseph’s proof works for all base fields k, q ∈ k× not a

root of unity, see [Yakb, Section 2.2]. For a fixed w, define Rw to be the localization of R

−1 Rw = R[Ew ] (4.2.5)

0 ⊂ Following Joseph [Jos95, Section 10.4.8] we define subalgebras Rw Rw by { } 0 λ λ −1 | ∈ ∈ ∗ Rw = Span cf (ew) λ P≥0, f V (λ) , (4.2.6)

0 The subalgebras Rw are invariant subalgebras of Rw with respect to the left action 3.2.5 of

Ki for all i. We remark that we do not need to take span in the right hand side of the above formula, cf. [Jos95, Section 10.4.8] or [Yakb, eq. (2.18)]. For λ1, λ2 ∈ P≥0, set

− − λ1 λ2 λ1 λ2 1 ∈ 0 ew = ew (ew ) Rw. (4.2.7)

µ1 µ2 It follows from (4.2.4) that this does not depend on the choice of λ1, λ2 and that ew ew =

µ1+µ2 ∈ 0 ew for all µ1, µ2 P . The algebra Rw is Q-graded by { } 0 λ −λ | ∈ ∈ ∗ (Rw)γ = cf ew λ P≥0, and f (V (λ) )γ+w(λ) .

57 + ⊂ For any Weyl group element y there is a corresponding Demazure module Uq V (λ)yλ

≥0 V (λ). These are extremal Uq -submodules of V (λ). Because the extremal weight spaces of

+ V (λ) are all one-dimensional, we may also write Uq Tyvλ for the Demazure module corre-

+ sponding to y. For example, note that Uq Tw0 vλ = V (λ) since Tw0 vλ is a lowest weight vector

≤0 − for V (λ). Similarly, we have Uq -submodules of V (λ) given by Uq Tyvλ. These upper and lower Demazure modules give rise to the upper and lower quantum Schubert cell ideals S(y) of R paramentrized by y ∈ W . These ideals are defined by { }  λ | ∈ ∈ ∗ ⊥  S (y) = Span cf λ P≥0 and f V (λ) such that f U Tyvλ .

0  0 By extension to Rw and contraction along Rw, we obtain the counterparts of S (y) in Rw. These ideals are given by { }  λ −λ | ∈ ∈ ∗ ⊥   −1 ∩ 0 Sw (y) = cf ew λ P≥0, f V (λ) such that f U Tyvλ = S (y)Ew Rw. (4.2.8)

Analogously to (4.2.6) one does not need to take a span in (4.2.8), see [Gor00, Yak10]. For

+ − γ ∈ Q≥0\{0}, we have dim U [w]γ = dim U [w]−γ. Denote this dimension by mγ, and choose

{ }mγ { }mγ + − dual bases uγ,i i=1 and u−γ,i i=1 of U [w]γ and U [w]−γ with respect to the Rosso–Tanisaki form, see [Jan96, Ch. 6]. The quantum R matrix corresponding to w is given by

∑ ∑mγ w + b − R = 1 ⊗ 1 + uγ,i ⊗ u−γ,i ∈ U ⊗ U , (4.2.9) γ=0̸ i=1 γ∈Q≥0

where U +⊗b U − is the completion of U +⊗U − with respect to the descending filtration [Lus93,

Section 4.1.1].

The following far-reaching theorem summarizes the representation theoretic approach to

SpecU −[w] via quantum function algebras and Demazure modules.

Theorem 4.2.2 (Yakimov). For any field k with q ∈ k× not a root of unity, any simple Lie

algebras g, and Weyl group elements w ∈ W , the following hold:

58 0 → − (i) The maps ϕw : Rw U [w] given by ( ) λ −λ λ w ∗ c e 7−→ c w ⊗ id (τ ⊗ id)R , with λ ∈ P≥0, f ∈ V (λ) (4.2.10) f w f,vλ

+ are well-defined surjective Q-graded algebra with ker ϕw = Sw (w).

(ii) For y ∈ W ≤w the ideals

+ − − Iw(y) = ϕw(Sw (w) + Sw (y)) = ϕw(Sw (y))

are distinct, H-invariant, completely prime ideals of U −[w]. Moreover, all H-prime

ideals of U −[w] are of this form.

≤w − (iii) The map y ∈ W 7→ Iw(y) ∈ H-SpecU [w] is an isomorphism of posets with respect to the Bruhat order on W ≤w and the inclusion order on H-SpecU −[w].

Part (i) is [Yaka, Theorem 2.6]. It was first proved in [Yak10] for another version of the

Hopf algebra Uq(g) equipped with the opposite coproduct, and a different braid group action.

w −1 Theorem 2.6 in [Yaka] used Twvλ in place of vλ = Tw−1 in equation (4.2.1) and Theorem 4.2.2

(i). The two formulations are equivalent since dim V (λ)wλ = 1 and Tw(V (λ)µ) = V (λ)wµ for all w ∈ W , λ ∈ P≥0, µ ∈ P . Parts (ii) and (iii) of Theorem 4.2.2 are proved in [Yakb, Theorem 3.1 (a)] relying on results of Gorelik [Gor00] and Joseph [Jos94]. These statements were earlier proved in [Yak10, Theorem 1.1 (a)-(b)] under slightly stronger conditions on k and q.

∈ λ ∈ − For λ P≥0 define the elements by,w U [w] by

( ) λ λ −λ by,w = ϕw ey ew ,

λ λ ∈ − and let by,w also denote the canonical images of by,w U [w]/Iw(y). We will keep this nota- tional convention throughout. For the sake of transparency, we through the definition

59 λ of by,w to obtain ( ) λ λ ⊗ Rw by,w = ey,wτ id ( ) ( ∑ ∑mγ ) λ ⊗ ⊗ ⊗ = fy (Twvλ)τ id 1 1 + uγ,i u−γ,i γ=0̸ i=1 γ∈Q≥0

− For all λ ∈ P≥0 and γ ∈ QS(w), these are nonzero normal elements of U [w]/Iw(y) by [Yakb, Theorem 3.1(b) and eq. (3.1)]:

λ −⟨(w+y)λ,γ⟩ λ ∈ − by,wx = q xby,w. for all x (U [w]/Iw(y))γ. (4.2.11)

The R-matrix commutation relations in R (see e.g. [BG02, Theorem I.8.15]) and eq. (4.2.4)

imply that for all λ1, λ2 ∈ P≥0

−1 λ1 λ2 −⟨λ1,λ2−y wλ2⟩ λ1+λ2 by,wby,w = q by,w .

− Thus, the multiplicative subset of U [w]/Iw(y) given by { } λ | ∈ ∈ k× By,w = kby,w λ P≥0 and k

− consists entirely of normal elements. Let Ry,w be the localized quotient of U [w]

− −1 Ry,w = (U [w]/Iw(y))[By,w]

This quotient is H-simple, its center Z(Ry,w) is a Laurent polynomial ring of dimension equal to dim ker(w + y), and the prime spectrum of U −[w] is partitioned into ⊔ − − SpecU [w] = SpecIw(y)U [w]. y∈W ≤w

− The strata SpecIw(y)U [w] are given by { } − P ∈ SpecU [w] | P ⊇ Iw(y) and P ∩ By,w = ∅ .

60 Moreover, extension and contraction establishes the homeomorphisms:

∼ ∼ = . = − SpecZ.(Ry,w) Spec.Ry,w SpecIw(y.)U [w].

We refer to [Yakb, Theorem 3.1 and Proposition 4.1] for details and proofs of the above

statements. The dimensions of the Laurent polynomial rings Z(Ry,w) were determined in [BCL, Yakb]. The previous results are compatible with the general Goodearl–Letzter H- stratification theory[GL00]. The above framework for SpecU −[w] is much more explicit. It

deals with specific H-prime ideals and localizations by small sets of normal elements.

b−a For all a, b, j ∈ [1, l] and m[a,b] = (ma, . . . , mb) ∈ N , define the symbols pj and pm by

(q−1 − q )−1 ij ij pj = − , and (4.2.12) qmj (mj 1)/2[m ] ! ij j ij { papa+1 . . . pb−1pb if a ≤ b

pm[a,b] = papa−1 . . . pb+1pb if a > b ∗ Proposition 4.2.3. Fix a Weyl group element w. For all λ ∈ P≥0 and f ∈ V (λ) the

w → − antihomomorphisms ϕw : R0 U [w] are explicitly given by ∑ − λ λ ⟨ m[l,1] w⟩ m[l,1] ϕw(cf ew ) = pm f, τ(E )vλ F (4.2.13) m∈Nl ∑ ⟨ m w⟩ m[l,1] = pm f, (τE) vλ F (4.2.14) m∈Nl

Proof. This follows from Theorem 4.2.2 (i) and the standard formula [Jan96, eqs. 8.30 (1)

and (2)] for the inner product of the pairs of monomials (iii) with respect to the the Rosso–

Tanisaki form.

61 Chapter 5 Main results and implications

5.1 Statement of main result

We recall from Section 4.1 that each length l reduced word i for a Weyl group element w gives a presentation of the quantum Schubert cell algebra U −[w] as a torsion free CGL extension − k U [w] = [Fβ1 ][Fβ2 ; σ2, δ2] ... [Fβl ; σl, δl]. (5.1.1)

By iteratively deleting derivations, we obtain a presentation of the associated Cauchon space

− k U [w] = [F β1 ][F β2 ; τ2] ... [F βl ; τl]. (5.1.2)

For brevity we write F i in place of F βi . In this section we explicitly describe of each of these quantum affine spaces using the antiisomorphisms from 4.2.2 (i); the generators of the

Cauchon spaces U −[w] are quantum minors or quotients of two quantum minors.

Given i = i . . . i , we define the successor function s: [1, l] ⊔ {∞} → [1, l] ⊔ {∞} by 1 l { min {k | j < k and ik = ij} if such a k exists s(j) = ∞ otherwise Example 5.1.1. Let i = 121321, then

s(1) = 3 s(2) = 5 s(3) = 6

s(4) = ∞ s(5) = ∞ s(6) = ∞

It is clear that s is locally nilpotent in the sense that for any j, we have sn(j) = ∞ for some n. Thus, we define the order of j ∈ [1, l] by

n |j| = max {n ∈ Z≥0 | s (j) ≠ ∞} . (5.1.3)

Example 5.1.2. Let i = 121321, then

62 |1| = 2 |2| = 1 |3| = 1

|4| = 0 |5| = 0 |6| = 0

ϖ Rw Recall the notation for the quantum minors ew , the R-matrix , the tau involution τ, and 0 → − Yakimov’s surjection ϕw : Rw U [w] from equations (4.2.2), (4.2.9), (3.1.1), and Theorem

− 4.2.2 (i). For each j ∈ [1, l] define the quantum minor ∆j ∈ U [w] by

ϖ ∆ = b ij (5.1.4) j w[1,j−1],w

This notation is extremely compact; upon unraveling it we obtain ( ) ( ) ϖ −ϖ ϖ ∆ = ϕ e ij e ij = e ij τ ⊗ id Rw j w w[1,j−1] w w[1,j−1],w ( )( ∑ ∑mγ ) ϖij = fw (T v )τ ⊗ id 1 ⊗ 1 + u ⊗ u− [1,j−1] w ϖij γ,i γ,i γ=0̸ i=1 γ∈Q≥0

We stress that “∆j” is abbreviated notation as ∆j is heavily dependent upon i. We have the following main theorem:

Theorem 5.1.3 (G., Yakimov). [G., Yakimov] Assume that k is an arbitrary base field,

q ∈ k× is not a root of unity, g is a simple Lie algebra, w ∈ W is a Weyl group element,

− and i is a reduced word for w. Then the generators F 1,..., F l of the Cauchon space U [w] are given by   c−1 −1 ≤ qij ∆s(j)∆j if s(j) l F j =  c−1 ∞ qij ∆j if s(j) = Theorem 5.1.3 is equivalent to the the following theorem to be proved in Section 5.1.2.

Theorem 5.1.4. In the setting of Theorem 5.1.3 the quantum minors ∆1,..., ∆l (5.1.4) are explicitly given by

|j| c | | ∆j = qij F s j (j) ... F j. (5.1.5)

63 As an illustration of the generality of this theorem, consider the following very special

situation: Given two non-negative integers m and n, let g be slm+n and set

m w = (12 . . . m + n) ∈ Sm+n (5.1.6)

By [MC10, Proposition 2.1.1] and [Yakb, Lemma 4.1] the algebras U [w] is then isomorphic

ϖij − to m × n quantum matrices Rq[Mm,n]. By [Yakb, Lemma 4.3] the elements by,w ∈ U [w]

correspond (under this isomorphism) to scalar multiples of quantum minors of Rq[Mm,n] for ∈ ≤w all j [1, m + n] and y in the Bruhat-order interval Sm+n (depending on the normalization from 4.2.1). The special case of this theorem for the algebras of quantum matrices Rq[Mm,n] is due to Cauchon [Cau03b]

5.1.1 Leading terms of quantum minors

− The presentation of U [w] as iterated Ore extension in the generators Fβ1 ,...,Fβl is by no means unique. The Levendorski˘i–Soibelman straightening law implies that we may adjoin

the generators in the opposite order as well. Translation from one presentation to the reverse

presentation will play a major role in our proof of Theorem 5.1.4 in Section 5.1.2. In Sections

5.1.1 and 5.1.2 we examine reversed iterated Ore extensions and prove a leading term result

for the elements ∆j. For a length l reduced expression w, we define the upper and lower truncated algebras

[ ] − k U [w][j,k] = Fβj ,Fβj+1 ,...,Fβk−1 ,Fβk

These algebras do, in general, depend on the reduced expression of w. The Levendorski˘i–

− − Soibelman straightening law gives that U [w][j,k] are subalgebras of U [w]. The truncated algebras are closely related to truncated reduced expressions (cf. (3.3.3)) as the following

proposition suggests.

64 Proposition 5.1.5. For all length l reduced expressions w and j ≤ k ∈ [1, l], we have

[ ] −1 − − T U w = U [w][j,k] w[1,j−1] [j,k]

Proof. Tracing through the definitions of the inherent symbols, we obtain

[ ] [ ] T −1 U − w = T −1 k F ,T F ,...,T F ,T F w − [j,k] w − ij ij ij+1 w[j,k−2] ik−1 w[j,k−1] ik [1,j 1] [[1,j 1] ] k −1 = T Fij ,Tw Fij+1 ,...,Tw Fi − ,Tw Fi w[1,j−1] [1,j] [1,k−2] k 1 [1,k−1] k [ ] k = Fβj ,Fβj+1 ,...,Fβk−1 ,Fβk

− = U [w][j,k]

−1 By noting that (w[1,j−1]) w[1,k] = w[j,k] we obtain the immediate and useful corollary

[ ] [ ] −1 − −1 −1 − T U (w − ) w = T U w (5.1.7) w[1,j−1] [1,j 1] [1,k] w[1,j−1] [j,k]

Towards the main result we prove the following theorem.

Theorem 5.1.6. For all fields k, q ∈ k× not a root of unity, simple Lie algebras g, length l

reduced expressions w, and j ∈ [1, l], we have   c − ̸ ∞ qij ∆s(j)Fβj mod U [w][j+1,l] s(j) = ∆j = (5.1.8)  c − ∞ qij Fβj mod U [w][j+1,l] s(j) =

Proof. For all k ∈ [1, l] we construct a twisted algebra Rk = τT (U k). Recall this algebra w[1,k−1] is isomorphic to U (sl )op, see (3.1). Set 1 ≤ k ≤ j − 1, and consider the Rk-submodule of qik 2

65 −1 V (ϖij ) generated by x = T vϖi . It is irreducible since w[j−1,1] j ( ) (τF )x = τ(T F ) T −1 v (5.1.9) βk w[1,k−1] k w − ϖij ( ) [j 1,1] = T −1 F T −1 v (5.1.10) w − k w − ϖij [k 1,1]( [j 1,1] ) = T −1 (T T −1 F )v (5.1.11) w − w[j−1,1] w − k ϖij [j 1,1] ( [k 1,1] ) −1 = T (Tw Fk)vϖi = 0; (5.1.12) w[j−1,1] [j−1,k] j

see equations (3.1.17) and (3.1.23). In the last equation we used that T (F ) ∈ U +. w[j−1,k] k

k Therefore, there is a R -module Vk such that

k ⊕ V (ϖij ) = R x Vk.

It follows that V is also U 0-stable. From this, equation (5.1.9), and the fact that dim V (ϖ ) = k ij w[1,j−1]ϖij 1 it follows that

ϖ ⟨f ij , (τE )v⟩ = 0, for all v ∈ V (ϖ ) and 1 ≤ k < j, (5.1.13) w[1,j−1] βk ij

recall (4.2.1).

j Next, we consider the R -submodule of V (ϖij ) generated by x. Using (i)–(ii), we obtain: ( ) ( )( ) (τE ) T −1 v = τ(T E ) T −1 v (5.1.14) βj w ϖij w[1,j−1] j w ϖij [j,1] ( )( [j,1] ) = T −1 E T −1 v w − j w ϖij [j 1,1]( [j,1]) −1 −1 = T EjT vϖi = x. (5.1.15) w[j−1,1] j j

Analogously one shows that ( ) −1 −1 (τEβj )x = 0 and τ(T Kj) x = q x. w[j−1,1] ij

Therefore, j k ⊕ k −1 R x = x T vϖi . w[j,1] j

66 Let v be in V (ϖij ). Using the preceeding fact, the complete reducibility of finite dimensional

i type one Uq-modules, and equation (5.1.14), we obtain :    ϖi ⟨ j ⟩ ϖ fw , v m = 1 ij m [1,j] ⟨fw , (τEβ ) v⟩ = [1,j−1] j  0 m > 1

In a similar way one proves that for all j < k ≤ min {l, s(j) − 1}

( ) k −1 k −1 −1 −1 R T vϖi = T vϖi and T vϖi = T vϖi . w[1,k−1] j w[1,k−1] j w[1,k−1] j w[1,k] j

From this one obtains that for all j < k ≤ min {l, s(j) − 1}

ϖ ϖ ϖ ⟨f ij , (τE )v⟩ = 0 and f ij = f ij . (5.1.16) w[1,k−1] βk w[1,k−1] w[1,k]

Proof of the Theorem 5.1.3. Equation (5.1.8) is deduced from from equations (5.1.13), (5.1.1),

and (5.1.16) as follows. Using (4.2.13), (5.1.13), and (5.1.1), we obtain:

∑ ϖi j m[l,j] −1 m[l,j] ∆ = p ⟨f , τ(E ) T − v ⟩F j m[j,l] w[1,j−1] w 1 λ m[j,l] ∑ ϖi j m[l,j+1] −1 m[l,j+1] − = qc p ⟨f , τ(E ) T − v ⟩F mod U [w] ij m[j+1,l] w[1,j] w 1 λ [j+1,l] m[j+1,l]

If s(j) ≤ l,we apply equation (5.1.16) to the above expression to obtain

∑ ϖi j m[l,sj] −1 m[l,s(j)] − ∆ = qc p ⟨f , τ(E ) T − v ⟩F F mod U [w] j ij m[s(j),l] w[1,s(j)−1] w 1 λ βj [j+1,l] m[s(j),l] c − = qij ∆s(j)Fβj mod U [w][j+1,l]

This proves equation (5.1.8). The proof of equation (5.1.6) is analogous, requiring only a

small modification of the last argument. It is left to the reader.

67 Starting from a reduced word i = i1, . . . , il for w ∈ W , one can construct a presentation

− of U [w] as an iterated Ore extension by adjoining the elements Fβ1 ,..., Fβl (recall (3.4.2)) in the opposite order. For all j ∈ [1, l] we have the Ore extension presentation

− − ← ← U [w][j,l] = U [w][j+1,l][Fβj ; σj , δj ], (5.1.17)

← ← ′ H where σj and δj are defined as follows. Let tj be an element of such that

′ −⟨ ⟩ βk βk,βj ≥ (tj) = q , for all k j

← ′ · (cf. (3.4.8) and (3.4.10)). We have that σj (x) = (tj x) in terms of the restriction of the H − ← − -action (3.4.9) to U [w][j+1,l]. The skew derivation δj of U [w][j+1,l] is defined by

← −⟨ ⟩ − − βj ,γ ∈ ∈ δj (x) = Fβj x q xFβj , where x (U [w][j+1,l])γ, and γ Q,

˘ ← cf. (3.4.11). The Levendorskii–Soibelman straightening law (3.4.3) implies that δj preserves

− ← ← U [w][j+1,l], where σl = id, and δl = 0.) Equations (iii) and (3.4.3) imply (5.1.17). Iterating − (5.1.17) with the convention U [w][l+1,l] = k leads to the presentation

− ← k ← ← ← ← U [w] = [Fβl ][Fβl−1 ; σl−1, δl−1] ... [Fβ1 ; σ1 , δ1 ], which is reverse to the presentation (5.1.1). It is straightforward to show U −[w]← is a torsion free CGL extension for the action (3.4.9). Theorem 5.1.6 gives that the quantum minors ∆j

− ← are in U [w][j,l] and provides a formula for its leading term as a left polynomial with respect to the Ore extension (5.1.17), for all j ∈ [1, l], cf. Section 4.1.

5.1.2 Proof of Theorem 5.1.4

We keep the notation for i, w, and l from the previous two subsections. For j ∈ [1, l]

consider the chain of extensions

− − − − k = U [w][j,j−1] ⊂ U [w][j,j] ⊂ U [w][j,j+1] ⊂ ... ⊂ U [w][j,l].

68 It follows from the Levendorski˘i–Soibelman straightening law (3.4.3) and the definition of the

H-action in equation (3.4.9) that the maps δk and σk from 3.4.4 (ii) preserve the subalgebra − − − ≤ ≤ ≤ U [w][j,k−1] of U [w[1,k−1]] = U [w][1,k−1] for all 1 j k l. For brevity set

| − | − jδk = δk U [w][j,k−1] and jσk = σk U [w][j,k−1] .

In particular, 1σk = σk and 1δk = δk. By 3.4.4 (ii) we have the presentation

− − ≤ ≤ ≤ U [w][j,k] = U [w][j,k−1][Fβk ; jσk, jδk], for 1 j k l.

− Using that U [w][j,j−1] = k, jσj = id, and jδj = 0, we obtain

− k U [w][j,l] = [Fβj ][Fβj+1 ; jσj+1, jδj+1] ... [Fβl ; jσl, jσl]. (5.1.18)

− It follows now from 3.4.4 that U [w][j,k] is a CGL extension. Since {0} is an H-prime ideal of

− U [w][j,k], we can apply the strong rationality theorem of Goodearl [BG02, Theorem II.6.4] to obtain

− H Z(Fract(U [w][j,l])) = k. (5.1.19)

where Z(A) denotes the center of an algebra A. As in Section 4.1, Fract(A) denotes the

H of fractions of a domain A, and (−) refers to the fixed point subalgebra with respect to the action of H. T 1  Fixing i as before, we denote by the quantum torus algebra generated by F 1 ,..., F l . ≥0 T The Q-grading on Uq induces a grading on . Equations (3.4.3) and (4.1.6) imply that

⟨βj ,βk⟩ F jF k = q F kF j, for all 1 ≤ j < k ≤ l. (5.1.20)

∈ T T 1 ≤ ≤ For j, k [1, l] denote by [j,k] the quantum subtorus of generated by F i for j i k. Using that ∈ − δk(Fβj ) U [w][k+1,j−1], by a simple induction argument one proves the following lemma:

69 Lemma 5.1.7. In the above setting, the following hold for all j ∈ [1, l]:

− ∈ T (i) Fβj F j [j+1,l]

− (ii) The generators for the Cauchon space of U [w][j,l] (with presentation as in equation

(5.1.18)) are F j, ..., F l. Therefore,

− − U [w][j,l] = U [w][j,l]

.

The lemma implies that

− − U [w][j,l] ⊂ T[j,l] ⊂ Fract(U [w][j,l]).

Therefore, the strong rationality result (5.1.19) gives that

Z(T[j,l])0 = k. (5.1.21)

Next, we apply a theorem of Berenstein and Zelevinsky [BZ05, Theorem 10.1], to obtain that

for all 1 ≤ j < k ≤ l there exists an integer njk such that

ϖi ϖi j ϖk njk ϖk j ew e = q e ew . [1,j−1] w[1,k−1] w[1,k−1] [1,j−1]

We remark that the setting of [BZ05] is for k = Q(q), but the proof of Theorem 10.1 in

[BZ05] only uses the R-matrix commutation relations in Rq[G] and the left and right actions

≥0 k ∈ k× of Uq on Rq[G] (from (3.2.5)), which are defined for all fields and q not a root of unity. Moreover, the R-matrix commutation relations in Rq[G] (see e.g. [BG02, Theorem

I.8.15]) imply that for all ν ∈ P , f ∈ V (λ)ν, and µ, λ ∈ P≥0

µ λ −⟨µ,λ+w−1ν⟩ λ µ + ewcf = q cf ew mod Q(w) .

70 0 → − Using (5.1.4) and the fact that the maps ϕw : Rw U [w] are antihomomorphisms by 4.2.2 ′ ∈ Z (i), we obtain that there is some njk for which

n′ ∆j∆k = q jk ∆k∆j, for all 1 ≤ j < k ≤ l. (5.1.22)

Proof of 5.1.4. By Lemma 5.1.7 (ii)

− U [w][j,l] ⊆ T[j,l], for all j ∈ [1, l].

Combining this, Theorem 5.1.6, and Lemma 5.1.7 (i), we obtain   c T ≤ qij ∆s(j)F j mod [j+1,l] if s(j) l (5.1.23) ∆j =  c T ∞ qij F j mod [j+1,l] if s(j) = (5.1.24)

We prove equation (5.1.5) by induction on j, from l to 1. By equation (5.1.24), ∆l −qblF l ∈ k.

Since ∆l is a homogeneous element of nonzero degree (equal to βl), this implies equation (5.1.5) for j = l.

Now assume that for some j ∈ [1, l − 1]

b |k| ∈ ∆k = qk F s|k|(k) ... F k for all k [j + 1, l]. (5.1.25)

If

|j| c | | ∆j = qij F s j (j) ... F j, (5.1.26) then we are done with the inductive step. Assume the opposite, that (5.1.26) is not satisfied.

Combining the inductive hypothesis with (5.1.23) and (5.1.24) (whichever applies for the particular j), we get that

|j| − c | | ∈ T ∆j qij F s j (j) ... F j [j+1,l]. (5.1.27)

It follows from equations (5.1.20), (5.1.22), and (5.1.25), that

mk ∆jF k = q F k∆j, for all k = j + 1, . . . , l

71 for some mj+1, . . . , ml ∈ Z. Quantum tori have bases consisting of Laurent monomials in their generators. By comparing the coefficients of F s|j|(j) ... F jF k in the two sides of the above equality and using (5.1.27), we get that

mk (F s|j|(j) ... F j)F k = q F k(F s|j|(j) ... F j), for all k = j + 1, . . . , l

for the same collection of integers mj+1, . . . , ml. From the last two equalities it follows that

−1 y := (F s|j|(j) ... F j) ∆j

commutes with F j+1,..., F l:

yF k = F ky, for all k = j + 1, . . . , l. (5.1.28)

Since (5.1.26) is not satisfied, (5.1.27) implies that

c ′ −1 ′ ∈ T \{ } y = qij + y F j for some y [j+1,l] 0 . (5.1.29)

But y commutes with itself and by (5.1.28) it commutes with y′ ≠ 0. Thus y also commutes

with F j. Combining this with (5.1.28) leads to the fact that y belongs to the center of T[j,l].

− Since ∆j is a homogeneous element of U [w] with respect to its Q-grading, (5.1.27) implies

y ∈ Z(T[j,l])0.

At the same time y∈ / k by (5.1.29), which contradicts the strong rationality result (5.1.21).

Thus equation (5.1.26) holds. This completes the proofs of the inductive step and the theo-

rem.

5.2 Unification of the two approaches to H-SpecU −[w] 5.2.1 Solutions of two questions of Cauchon and M´eriaux

In this section we establish a dictionary between the representation theoretic and ring

theoretic approaches to H-SpecU −[w], see Section 4.2 and Section 4.1. Theorem 5.2.5 ex-

plicitly describes the behavior of all H-prime ideals Iw(y), from Theorem 4.2.2, under the

72 deleting derivations. We also describe the Cauchon diagrams of all ideals Iw(y) and use this to resolve [MC10, Question 5.3.3] of Cauchon and M´eriaux; see Theorem 5.2.1. We use the

combination of Theorems 4.2.2 and 5.2.1 to give an independent and elegant proof of the

Cauchon and M´eriauxclassification in our proof of Theorem 5.2.3. Finally, we also settle

[MC10, Question 5.3.2] of Cauchon and M´eriaux, solving the poset containment problem in

the classification of 4.1.6, see 5.2.4.

Recall Section 3.3 and Corollary 3.3.5 for definitions and details of the left positive diagram

bijection

+ ≤w L D D : .W . L.w D.l

and its counterpart the Cauchon diagram bijection

∼ − φ . − = ∼ C : H-Spec. U [w] H-Spec.U [w] P.l D.l.

+ ∈ 0 Also, recall the quantum Schubert cell ideals Sw (y) Rw and the algebra isomorphism ∼ 0 + = − ϕw : Rw/Sw (w) . . .U [w]. For a fixed Weyl group element w, set

{ } S− − + | ∈ ≤w w = Sw (y) + Sw (w) y W .

By the Yakimov’s classification Theorem, 4.2.2 iii, the map ϕw induces a poset isomorphism

− ≤w Sw S− . ϕw H − Iw : W. w. -Spec.U [w]

The following main theorem summarizes and relates all known approaches to analyzing the

H-prime spectrum of U −[w]:

Theorem 5.2.1 (G., Yakimov). For any field k with q ∈ k× not a root of unity, simple

Lie algebra g, weyl group element w, and reduced word i for w representing w, we have a

73 commutative diagram

D ∼ C − Sw. P.l D.l . H-Spec.U [w] D + D . ϕw Iw L ≤w S− .w L. W . . − w. Sw

The left hand square commutes by the analysis conducted in Section 3.3, and commu-

tativity of the right hand square is proved in subsection 5.2.2. This main theorem is the

bridge between the representation theoretic and the ring theoretic approach to analyzing

H-SpecU −[w]. In particular, the theorem implies

+ C(Iw(y)) = D (y). (5.2.1)

The following is, therefore, an immediate consequence of Theorem 5.2.1. It settles Question

5.3.3 of Cauchon and M´eriaux[MC10]; the Cauchon–M´eriaux[MC10] and Yakimov [Yak10]

classifications of H-SpecU −[w] of described in 4.1.6 and Theorem 4.2.2 coincide.

Theorem 5.2.2 (G., Yakimov). For all base fields k, q ∈ k× not a root of unity, simple Lie algebras g, Weyl group elements w, and reduced words i for w,

≤w Iw(y) = JD+(y) for all y ∈ W . (5.2.2)

−1 + −1 + Proof. By Theorem 5.2.1 we have Iw(y) = C (D (y)), but C (D (y)) = JD+(y) by defini- tion. Therefore, we obtain the result.

The Cauchon–M´eriauxclassification [MC10]—a result whose abridged version we restate

for convenience—is now a simple corollary of Theorem 5.2.1.

Theorem 5.2.3. (Cauchon–M´eriaux, [MC10]) The H-prime ideals of U −[w] are the ideals

≤w JD+(y) for the elements y ∈ W .

74 Proof. By an application of Yakimov’s classification Theorem 4.2.2 ii and our unification

result, Theorem 5.2.2, we have

− ≤w ≤w H-SpecU [w] = {Iw(y) | y ∈ W } = {JD+(y) | y ∈ W }.

Finally, the next theorem answers Question 5.3.2 of Cauchon and M´eriaux[MC10].

Theorem 5.2.4 (G., Yakimov). For all base fields k, q ∈ k× not a root of unity, simple Lie algebras g, Weyl group elements w, and reduced words i for w, the map W ≤w . . . H-SpecU −[w]

y . . .JD+(y)

is an isomorphism of posets with respect to the Bruhat order and inclusion of ideals.

Proof. By Theorem 4.2.2 iii, the map sending y to Iw(y) is a poset isomorphism. Thus, by Theorem 5.2.2, the result follows.

We provide a complete description of the behavior of the ideals Iw(y) under the deleting derivation procedure. Recall the definition of leading part Je of an ideal in an Ore extension.

According to Proposition 4.1.4, Cauchon’s method relies on taking leading parts of ideals or

contracting ideals. For the remainder of this section assume that i = (i1, . . . , il) is a reduced word for w ∈ W . It follows immediately that

w[1,l−1] = wsil , (5.2.3)

and Theorem 3.4.4 (i) and (ii) imply

− − ⊂ − − − U [w[1,l−1]] = U [w[1,l−1]] U [w] and U [w] = U [w[1,l−1]][Fβl ; σl, δl]. (5.2.4)

75 Theorem 5.2.5 (G., Yakimov). Let k be an arbitrary base field, q ∈ k× not a root of unity, g a simple Lie algebra, w ∈ W a Weyl group element, and i = (i1, . . . , il) a reduced word

∈ ≤w − − for w. Then the following hold for all y W inside U [w] = U [w[1,l−1]][Fβl ; σl, δl] (from equation (5.2.4)):

(i) If l∈ / D+(y), then I](y) = I (y) w w[1,l−1]

(ii) If l ∈ D+(y), then I (y) ∩ U −[w ] = I (ys ) w [1,l−1] w[1,l−1] il

We prove Theorem 5.2.1 using 5.2.5 in this subsection, and we establish 5.2.5 in Section

5.2.3–5.2.4. With the goal of proving Theorem 5.2.1, we first establish an auxiliary lemma.

Lemma 5.2.6. In the setting of Theorem 5.2.5, if y is ssuch that l ∈ D+(y), then

T v ∈/ U −T v . (5.2.5) w[1,l−1] ϖil y ϖil

Proof. We proceed by induction on ℓ(w). If l = 1, then T v = v Now, since 1 ∈ D+(y) wsi1 ϖi1 ϖi1

eliminates the possibility that y = id. Thus, y(ϖi1 ) < ϖi1 and the result follows. Now assume the statement for ℓ(w) = l − 1, and suppose that (5.2.5) does not hold, that

is,

T v ∈ U −T v . (5.2.6) w[1,l−1] ϖil y ϖil

There are two possibilities: 1 ∈ D+(y) or 1 ∈/ D+(y).

+ Case (A): 1 ∈ D (y). We begin by noting that i[2,l] = (i2, . . . , il) is a reduced word for D+ si1 w. By the definition of positivity (y), we have

D+(y) D+(y) y = si1 w[2,l] > w[2,l] = si1 y. (5.2.7)

≤ D+ D+ \{ } Moreover, we have si1 y si1 w and (si1 y) = (y) 1 . Recall the definition (3.1) of i ∈ the subalgebras Uq of Uq(g) for i [1, r]. Equation (5.2.6), (5.2.7) and [Jos95, Lemma 4.4.3

76 (iii)–(iv)] imply

− − − T v ∈ U i1 T v ⊆ U i1 U T v = U U i1 T v = U T v , si1 w[1,l−1] ϖil w[1,l−1] ϖil y ϖil y ϖil si1 y ϖil

which contradicts the inductive hypothesis for si1 y, si1 w, and i[2,l]. Case (B)‘: 1 ∈/ D+(y). The argument in this case is similar to the previous one. From the left positivity of the index set D+(y) we have

D+(y) D+(y) si1 y = si1 w[2,l] > w[2,l] = y. (5.2.8)

Furthermore, y < s w and D+ (y) = D+(y). Equations (5.2.6), (5.2.8) and [Jos95, Lemma i1 i[2,l] 4.4.3 (iii)–(iv)] imply

− − − T v ∈ U i1 T v ⊆ U i1 U T v = U U i1 T v = U T v , si1 w[1,l−1] ϖil w[1,l−1] ϖil y ϖil y ϖil y ϖil

but this contradicts the inductive hypothesis on y, si1 w, and i[2,l]. ∑ One can similarly show T v ∈/ U −T v for λ ∈ Z ϖ , which follows from w[1,l−1] λ y λ α∈Π >0 α ̸≤ [Jos95, Lemma 4.4.5] and the simple fact that y w[1,l−1]. 5.2.2 Proof of Theorem 5.2.1

Before proceeding, we establish the following lemma to aid our inductive arguments:

Lemma 5.2.7. Suppose y is a left positive subexpression for y ∈ W ≤w with diagram D.

≤ ∈ ∈ wsil D (i) If l / D, then D is left positive diagram for y W . Thus, ysil = w[1,l−1].

′ ≤ ∈ \{ } ∈ wsil (ii) If l D, then D = D l is a left positive diagram for ysil W . Therefore,

D′ y = w[1,l−1].

+ D D ∈ − Proof. Let D = D (y). By assumption sij w[j+1,l] > w[j+1,l] for all j [1, l 1]. Part i: If l∈ / D, then we obtain for all j ∈ [1, l − 2] that D satisfies

D D D D sij w[j+1,l−1] = sij w[j+1,l] > w[j+1,l] = w[j+1,l−1],

77 Part ii: If l ∈ D, then for all j ∈ [1, l − 1] we have that D′ satisfies

D′ D D D′ sij w[j+1,l−1]sil = sij w[j+1,l] > w[j+1,l] = w[j+1,l−1]sil .

Upon right multiplication by sil , we obtain the result.

Proof of Theorem 5.2.1. We prove Theorem 5.2.1 by induction on the length ℓ(w). The case

ℓ(w) = 0 is trivial. Therefore, we assume the hypothesis for ℓ(w) = l − 1. Note that

i[1,l−1] = (i1, . . . , il−1)

is a reduced word for the the reduced expression w[1,l−1] = wsil . In the setting of Section 4.1, × xl = xl. 5.1.3 implies that, for some pl ∈ k ,

ϖ F = p ∆ = p b il . βl l l l w[1,l−1],w

Again, we have two cases: (A) l∈ / D+(y) and (B) l ∈ D+(y).

∈ D ≤ Case (A): l / D. It follows from Lemma 5.2.7 (i) that y = w[1,l−1] w[1,l−1] and D+ (y) = D. By the inductive hypothesis, we therefore have i[1,l−1]

C(I (y)) = D. (5.2.9) w[1,l−1]

ϖ Recall from Section 4.2 that b il ∈/ I (w ); see [Yakb, Theorem 3.1 (b)]. Moreover, w[1,l−1],w w [1,l−1] ≤ ⊆ y w[1,l−1] implies Iw(y) Iw(w[1,l−1]) by 4.2.2 ii. Thus,

ϖ F = p b il ∈/ I (y) ⊂ I (w ). βl l w[1,l−1],w w w [1,l−1]

] Now, we can apply Proposition 4.1.4 i to the J = Iw(y). By Theorem 5.2.5 i, Iw(y) = I (y). Finally, by combining Proposition 4.1.4 i and equation (5.2.9) we obtain w[1,l−1]

C(I (y)) = C(I (y)) = D. w w[1,l−1]

78 ∈ D D′ ′ Case (B): l D. By Lemma 5.2.7 (i), we have ysil = w sil = w[1,l−1] and hence D = D+ (ys ). The inductive hypothesis, applied to ys ≤ w , implies i[1,l−1] il il [1,l−1]

C(I (ys )) = D′. (5.2.10) w[1,l−1] il

Lemma 5.2.6 gives that T v ∈/ U −T v ; therefore, by definition f ∈ (U −T v )⊥, w[1,l−1] ϖil y ϖil w[1,l−1],ϖil y ϖil ϖ and we have F = p b il ∈ I (y). Now we are in a position to apply Proposition 4.1.4 ii βl l w[1,l−1],w w (again) to J = I (y). Theorem 5.2.5 ii implies I (y) ∩ U −[w ] = I (ys ). It follows w w [1,l−1] w[1,l−1] il from Proposition 4.1.4 ii and equation (5.2.10) that

C(I (y)) = C(I (ys )) ⊔ {l} = D′ ⊔ {l} = D. w w[1,l−1] il

5.2.3 Proof of Theorem 5.2.5 (i)

Let U −[w] have the prescribed Ore extension presentation

− − U [w] = U [w[1,l−1]][Fβl ; σl, δl].

The following result, important in its own right, will help us prove Part i of the theorem.

Theorem 5.2.8 computes the leading term of the right polynomial in Fβl

∑N ϕ (cλe−λ) ∈ F mU −[w ], w f w βl [1,l−1] m=0 where N = ⟨λ, α∨⟩. We remark that this result is strongly analogous to Theorem 5.1.6. il

Proposition 5.2.8 (G., Yakimov). For all base fields k, q ∈ k× not a root of unity, Weyl

group elements w ∈ W , reduced words i = (i1, . . . , il) for w, dominant integral weights

∗ λ ∈ P≥0, and f ∈ V (λ) , we have

(q−1 − q )N λ −λ il il N λ −λ ϕw(cf ew ) = − Fβ ϕw (cf ew ) + lower order terms. qN(N 1)/2 l [1,l−1] [1,l−1] il

79 il Proof of Theorem 5.2.8. Recall (3.1). The vector vλ is a highest weight vector for U of

highest weight Nϖil . Lemma 3.1.5 (i) and (ii) (respectively) imply that for all m > N

1 EN T −1v = EN F N v = [N] !v il il λ il il λ il il [N]il ! and

EmT −1v = 0. il il λ

Therefore, for all m > N ( )( ) N −1 −1 N −1 −1 (τEβ ) T −1 vλ = T (E ) T T vλ l w w[l−1,1] il w[l−1,1] il ( ) −1 N −1 = T E T vλ w[l−1,1] il il

−1 = [N]i !T vλ l w[l−1,1] and similarly

m −1 (τEβl ) Tw−1 vλ = 0.

w → − Using the formula (4.2.13) for the antihomomorphism ϕw : R0 U [w], we obtain that for

∗ all λ ∈ P≥0, f ∈ V (λ)

−1 − N ∑ − (qi qil ) − λ λ l ⟨ m[1,l−1] 1 ⟩ N m[l−1,1] ϕw(c e ) = pm − f, τ(E )T vλ F F f w N(N−1)/2 [1,l 1] w[l−1,1] βl q l−1 il m[1,l−1]∈N − (q−1 − q )N N∑1 il il N λ −λ m − = − Fβ ϕw (cf ew ) mod Fβ U [w[1,l−1]]. qN(N 1)/2 l [1,l−1] [1,l−1] l il m=0

We proceed by showing I (y) ⊂ I](y) by Theorem 5.2.8. We will then will compare w[1,l−1] w − − ] the Gelfand–Kirillov dimensions of U [w]/Iw(y) and U [w[1,l−1]]/Iw(y) to obtain the reverse inclusion

80 Proof of Theorem 5.2.5 (i). In the proof of Theorem 5.2.1 we showed that l∈ / D implies ∈ H − Fβl / Iw(y). Now, Iw(y) is an -invariant completely prime ideal of U [w]. Applying Propo- ] sition 4.1.4 i to J = Iw(y), we see that Iw(y) is an H-invariant, completely prime ideal of

− ′ ≤ ∈ wsα U [w[1,l−1]]. By Theorem 4.2.2 i, there is a y W such that

I](y) = I (y′). w w[1,l−1]

− ⊥ ∗ Proceeding, we let λ ∈ P≥0 and f ∈ (U Tyvλ) ⊂ V (λ) . By definition and by Theorem 5.2.8, respectively, we have

λ −λ λ −λ ] ϕw(c e ) ∈ Iw(y) and ϕw (c e ) ∈ Iw(y). f w [1,l−1] f w[1,l−1]

Therefore, I (y) ⊂ I](y), which is true if and only if y ≤ y′. To prove the opposite w[1,l−1] w containment note that by (iii),

− − ] GKdim(U [w]/Iw(y)) = GKdim(U [w[1,l−1]]/Iw(y)) + 1

− ′ = GKdim(U [w[1,l−1]]/Iw(y )) + 1.

On the other hand, [Yakb, Theorem 5.8] gives that

− − − ′ − − ′ GKdim(U [w]/Iw(y)) = l ℓ(y) and GKdim(U [w[1,l−1]]/Iw(y )) = l 1 ℓ(y ).

Therefore, ℓ(y′) = ℓ(y), but since y ≤ y′ we must have y′ = y, which yields the result

I](y) = I (y). w w[1,l−1]

5.2.4 Proof of Theorem 5.2.5 (ii) ∩ − A straightforward computation of the contraction Iw(y) U [w[1,l−1]] is complicated and impractical. We instead deduce

≤ − ′ ′ w − I (y) ∩ U [w − ] = I (y ) for some y ∈ W [1,l 1] w [1,l 1] w[1,l−1]

81 and analyze the homogeneous, nonzero P -normal elements in

− ∼ − ∩ − U [w]/Iw(y) = U [w[1,l−1]]/(Iw(y) U [w[1,l−1]]) (5.2.11)

′ to conclude y = ysil . Observe there is a natural inclusion P . . .H of the weight lattice given by

⟨ ⟩ ⟨ ⟩ λ . . .(q λ,α1 , . . . , q λ,αr ).

− − This in turn gives rise to a P -action on Uq(g), U [w], and U [w]/Iw(y), given by

⟨λ,γ⟩ λ · x = q x for x ∈ (Uq(g))γ.

Suppose a group G acts on a ring R. An element r of R is called G-normal or equivariantly

normal with respect to G if there exists g ∈ G such that

rx = (g · x)r for all x ∈ R.

∈ ≤w ∈ λ ∈ − For all y W and λ P , the elements by,w U [w]/Iw(y) are nonzero, homogeneous, and P -normal by equation (4.2.11). Alternatively, The next proposition is a result in the

opposite direction concerning the possible weights of all homogeneous P -normal elements of

− U [w]/Iw(y).

Proposition 5.2.9 (G., Yakimov). For all base fields k, q ∈ k× not a root of unity, Weyl

≤w − group elements y ∈ W , and nonzero, homogeneous, P -normal elements u ∈ U [w]/Iw(y),

− there exists ρ ∈ (1/2)P such that (w − y)ρ ∈ QS(w), u ∈ (U [w]/Iw(y))(w−y)ρ, (w + y)ρ ∈ P , and

−⟨(w+y)ρ,γ⟩ − ux = q xu, for all γ ∈ Q and x ∈ (U [w]/Iw(y))γ.

− Proof. Let u ∈ (U [w]/Iw(y))β be a homogeneous P -normal element such that

⟨ρ′,γ⟩ − ux = q xu for all γ ∈ Q and x ∈ (U [w]/Iw(y))γ (5.2.12)

82 ′ for some ρ ∈ P ; here β is an element in the support lattice QS(w). Equations (4.2.11) and

(5.2.12) imply that for all λ ∈ P≥0

λ −⟨(w+y)λ,β⟩ λ by,wu = q uby,w

−⟨(w+y)λ,β⟩ ⟨ρ′,(w−y)λ⟩ λ = q q by,wu.

× − Now q ∈ k is not a root of unity and U [w]/Iw(y) is a domain, so the preceding equality is only possible if

′ −⟨(w + y)λ, β⟩ + ⟨ρ , (w − y)λ⟩ = 0 for all λ ∈ P≥0.

Therefore,

−1 −1 ′ ⟨wλ, (wy + 1)β⟩ + ⟨wλ, (wy − 1)ρ ⟩ = 0 for all λ ∈ P≥0, that is,

(wy−1 + 1)β = (wy−1 − 1)(−ρ′) = 0.

By a standard argument for Cayley transforms (see[Yaka, Theorem 3.6])), we obtain that there exits ρ ∈ QΠ satisfying

β = (wy−1 − 1)yρ = (w − y)ρ and − ρ′ = (wy−1 + 1)yρ = (w + y)ρ (5.2.13)

Thus, we have

− (w − y)ρ ∈ QS(w),u ∈ (U [w]/Iw(y))(w−y)ρ, and (w + y)ρ ∈ P.

Combining the equalities in (5.2.13) and solving for ρ gives

1 ρ = w−1(β − ρ′). 2

Finally, substituting −(w + y)ρ for ρ in (5.2.12) gives the final claim

−⟨(w+y)ρ,γ⟩ − ux = q xu for all γ ∈ Q and x ∈ (U [w]/Iw(y))γ.

83 ∈ ∈ Proof of 5.2.5 (ii). In the proof of Theorem 5.2.1, we saw that l D implies Fβl Iw(y).

− Recall equation (5.2.3). Since Iw(y) is a H-invariant, completely prime ideal of U [w], we ∩ − H − have that Iw(y) U [w[1,l−1]] is a -invariant completely prime ideal of U [w[1,l−1]]. It follows ′ ≤w from Theorem 4.2.2 (i) that there is some y ∈ W [1,l−1] such that

− ′ I (y) ∩ U [w − ] = I (y ). w [1,l 1] w[1,l−1]

The H-eigenvectors of Uq(g) are precisely the homogeneous vectors of the Q-grading of Uq(g). Thus, by Proposition 4.1.5 ii we have a Q-graded algebra isomorphism

′ − ′ ∼ − U := U [w − ]/I (y ) = U [w]/I (y). [1,l 1] w[1,l−1] w

Denote the support of the Q-grading of the above algebras:

′ Z{ ∈ | ′ ̸ } ⊆ Q = γ Q Uγ = 0 Q.

∈ λ For λ P equation (4.2.11) implies that by,w is a nonzero, homogeneous, P -normal element

′ of U(w−y)λ and λ −⟨(w+y)λ,γ⟩ λ ∈ ′ ∈ ′ by,wx = q xby,w for all γ Q and x Uγ. (5.2.14)

Applying Proposition 5.2.9 to the algebra U ′ = U −[w ]/I (y′) and the homogeneous, [1,l−1] w[1,l−1] λ ′ ∈ λ ∈ ′ P -normal element by,w, we have there exists µ (1/2)P such that by,w U − ′ , (w[1,l−1] y )µ ′ ∈ (w[1,l−1] + y )µ P , and

−⟨(w +y′)µ,γ⟩ ′ ′ λ [1,l−1] λ ∈ ∈ by,wx = q xby,w for all γ Q and x Uγ. (5.2.15)

As before we use the fact that q ∈ k× is not a root of unity, that U ′ is a domain, and we combine equations (5.2.14) and (5.2.15) to obtain that for all γ ∈ Q′

− − ′ ⟨ ⟩ ⟨ ′ ⟩ (w y)λ = (w[1,l−1] y )µ and (w + y)λ, γ = (w[1,l−1] + y )µ, γ . (5.2.16)

84 Therefore, for all γ ∈ Q′, we have

⟨wλ, γ⟩ = ⟨(w − y)λ + (w + y)λ, γ⟩

⟨ − ′ ′ ⟩ = (w[1,l−1] y )µ + (w[1,l−1] + y )µ, γ ⟨ ⟩ = w[1,l−1](µ), γ .

∈ ̸ ν ∈ ′ − ′ ∈ ′ For all ν P≥0 Since 0 = bw ,y′ U − ′ , we have (w − y )ν Q ; hence, by [1,l−1] (w[1,l−1] y )ν [1,l 1] the previous string of equalities,

⟨ − − ′ ⟩ w[1,l−1](sil λ µ), (w[1,l−1] y )ν = 0,

that is, ⟨ ′ − − ′ ⟩ (y w[1,l−1])(sil λ µ), y ν = 0.

′ − ′ − Thus, (y w[1,l−1])µ = (y w[1,l−1])sil λ. The first part of (5.2.16) then gives

− − ′ (w y)λ = (w[1,l−1] y )sil λ.

′ ∈ ′ and so yλ = y sil (λ) for all λ P≥0. This, however, is only possible if y = ysil ; hence,

− I (y) ∩ U [w − ] = I (ys ), w [1,l 1] w[1,l−1] il and our proof is complete.

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88 + Appendix: Relations in U [w0] for type A3

+ Consider the A3 quantum Schubert cells Uq [w0] for the lexicographically minimal reduced decomposition w0 = s1s2s1s3s2s1 = s121321. From this we obtain an order on ∆+ given by

α < α + β < β < α + β + γ < β + γ < γ.

Recall that a theorem of Levendorski˘i and Soibelman gives that

∑ −⟨ ⟩ n n − X X − q βi,βj X X = c X i+1 ...X j 1 for j > i. (A.17) βj βi βj βj n βi+1 βj−1

In general the right hand side of (A.17) is unknown. Abbreviate Xβi to Xi. We investigate a very specific case with further analysis to come in forthcoming publications. Let us note that

the relation immediately gives that the root vectors Xi and Xi+1 commute if ⟨βi+1, βi⟩ = 0 and q-commute otherwise. Due to the prevalence of relations of the form −XY +q−1YX = Z,

−1 we denote the q-commutator by [Xi,Xj]q = XiXj − q XjXi. In particular [Xi,Xj]1 is the usual commutator bracket. Note that [X,Y ]q is related to [Y,X]q by

−1 −[X,Y ]q = [−X,Y ]q = [X, −Y ]q = q [Y,X]q−1 .

+ Consider the A3 quantum Schubert cells Uq [w0] for the lexicographically minimal reduced decomposition w0 = s1s2s1s3s2s1 = s121321. From this we obtain an order on ∆+ given by

α < α + β < β < α + β + γ < β + γ < γ.

89 To this order we associate the generators

Eα = E1

−1 Eα+β = −E1E2 + q E2E1

= −[E1,E2]q

Eβ = E2

−1 −1 −2 Eα+β+γ = E1E2E3 − q E2E3E1 − q E3E1E2 + q E3E2E1

= [E1, [E2,E3]q]q = [[E1,E2]q,E3]q

−1 Eβ+γ = −E2E3 + q E3E2

= −[E2,E3]q

Eγ = E3

Note that we are using the braid group action in [[BG02] Section I.6.7 equation (6)] and not the previously one from (3.1.12). We construct a diagram to encode the following relations among these generators:

X. . Y. X. . Z. X. . Y.

. . . . . Y. . . .

. −[X,Y.]q = 0 . . −[X,Y.]q = Z . . [X,Y.] = 0 .

. YX =. qXY . YX. = qXY. + qZ. . YX =. XY .

90 X. Y. [.X,W ] = (q. − q−1)YZ

. Z. W. XW. + q−1YZ .= qY Z + WX

+ Here are explicit relations (in positive form) for U [s121321]:

qX2X1 = X1X2 qX3X2 = X2X3 qX4X2 = X2X4

qX5X3 = X3X5 qX5X4 = X4X5 qX6X5 = X5X6

−1 −1 q X3X1 = X1X3 + X2 q X6X3 = X3X6 + X5

−1 −1 q X5X1 = X1X5 + X4 q X6X2 = X2X6 + X4

−1 X2X5 + q X3X4 = X5X2 + qX3X4

+ For any subset D of {1,..., 6}, we can consider the subalagebra of U genrated by {Xi | ∈ } + { } + i D . Denote this by UD . More details later. If D = 1, 2, 4 , then UD is a quantum { } + affine space on three generators. If D = 1, 2, 3 , then UD is a q-deformed version of the { } + × Heisenberg Lie algebra. If D = 2, 3, 4, 5 , then UD is 2 2 quantum matrices. There are obvious relation subdiagrams corresponding to these subalgebras.

We conclude by providing the relation diagrams for all sixteen reduced decompositions of

− w0. These diagrams will play a major role in our future results on U [w].

91 E.α Eα.+β Eα+.β+γ

. . E.β Eβ.+γ

. . E.γ

FIGURE A.1. Relation diagram for i = 121321

X.1 X.2 X.4

. . X.3 X.5

. . X.6

FIGURE A.2. Relation diagram for i = 121321

X.1 X.2 X.3

. . X.4 X.5

. . X.6

FIGURE A.3. Relation diagram for i = 123121

92 X.1 X.2 X.3

. . X.6 X.5

. . X.4

FIGURE A.4. Relation diagram for i = 123212

X.1 X.4 X.3

. . X.6 X.5

. . X.2

FIGURE A.5. Relation diagram for i = 132312

X.1 X.5 X.3

. . X.6 X.4

. . X.2

FIGURE A.6. Relation diagram for i = 132132

93 X.3 X.2 X.4

. . X.1 X.5

. . X.6

FIGURE A.7. Relation diagram for i = 212321

X.5 X.2 X.4

. . X.1 X.3

. . X.6

FIGURE A.8. Relation diagram for i = 213231

X.6 X.2 X.4

. . X.1 X.3

. . X.5

FIGURE A.9. Relation diagram for i = 213213

94 X.6 X.3 X.4

. . X.1 X.2

. . X.5

FIGURE A.10. Relation diagram for i = 231213

X.5 X.3 X.4

. . X.1 X.2

. . X.6

FIGURE A.11. Relation diagram for i = 231231

X.6 X.5 X.4

. . X.1 X.2

. . X.3

FIGURE A.12. Relation diagram for i = 232123

95 X.2 X.5 X.3

. . X.6 X.4

. . X.1

FIGURE A.13. Relation diagram for i = 312132

X.2 X.4 X.3

. . X.6 X.5

. . X.1

FIGURE A.14. Relation diagram for i = 312312

X.4 X.5 X.3

. . X.6 X.2

. . X.1

FIGURE A.15. Relation diagram for i = 321232

96 X.6 X.5 X.3

. . X.4 X.2

. . X.1

FIGURE A.16. Relation diagram for i = 321323

X.6 X.5 X.4

. . X.3 X.2

. . X.1

FIGURE A.17. Relation diagram for i = 323123

97 Vita

Joel Benjamin Geiger was born in Houma, Louisiana where he earned his high school diploma from H.L. Bourgeois High School. In May 2005, he obtained his Bachelor of Science degree in Mathematics at Nicholls State University in Thibodaux, Louisiana. In August 2008

Geiger initiated his graduate studies at Louisiana State University and earned his Master of

Science in Mathematics in May 2010. He is currently is a candidate for the degree of Doctor of Philosophy in Mathematics to be awarded in August 2013.

98