K-Orbits on the Flag Variety and Strongly Regular Nilpotent Matrices
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PCMI Summer 2015 Undergraduate Lectures on Flag Varieties 0. What
PCMI Summer 2015 Undergraduate Lectures on Flag Varieties 0. What are Flag Varieties and Why should we study them? Let's start by over-analyzing the binomial theorem: n X n (x + y)n = xmyn−m m m=0 has binomial coefficients n n! = m m!(n − m)! that count the number of m-element subsets T of an n-element set S. Let's count these subsets in two different ways: (a) The number of ways of choosing m different elements from S is: n(n − 1) ··· (n − m + 1) = n!=(n − m)! and each such choice produces a subset T ⊂ S, with an ordering: T = ft1; ::::; tmg of the elements of T This realizes the desired set (of subsets) as the image: f : fT ⊂ S; T = ft1; :::; tmgg ! fT ⊂ Sg of a \forgetful" map. Since each preimage f −1(T ) of a subset has m! elements (the orderings of T ), we get the binomial count. Bonus. If S = [n] = f1; :::; ng is ordered, then each subset T ⊂ [n] has a distinguished ordering t1 < t2 < ::: < tm that produces a \section" of the forgetful map. For example, in the case n = 4 and m = 2, we get: fT ⊂ [4]g = ff1; 2g; f1; 3g; f1; 4g; f2; 3g; f2; 4g; f3; 4gg realized as a subset of the set fT ⊂ [4]; ft1; t2gg. (b) The group Perm(S) of permutations of S \acts" on fT ⊂ Sg via f(T ) = ff(t) jt 2 T g for each permutation f : S ! S This is an transitive action (every subset maps to every other subset), and the \stabilizer" of a particular subset T ⊂ S is the subgroup: Perm(T ) × Perm(S − T ) ⊂ Perm(S) of permutations of T and S − T separately. -
A Quasideterminantal Approach to Quantized Flag Varieties
A QUASIDETERMINANTAL APPROACH TO QUANTIZED FLAG VARIETIES BY AARON LAUVE A dissertation submitted to the Graduate School—New Brunswick Rutgers, The State University of New Jersey in partial fulfillment of the requirements for the degree of Doctor of Philosophy Graduate Program in Mathematics Written under the direction of Vladimir Retakh & Robert L. Wilson and approved by New Brunswick, New Jersey May, 2005 ABSTRACT OF THE DISSERTATION A Quasideterminantal Approach to Quantized Flag Varieties by Aaron Lauve Dissertation Director: Vladimir Retakh & Robert L. Wilson We provide an efficient, uniform means to attach flag varieties, and coordinate rings of flag varieties, to numerous noncommutative settings. Our approach is to use the quasideterminant to define a generic noncommutative flag, then specialize this flag to any specific noncommutative setting wherein an amenable determinant exists. ii Acknowledgements For finding interesting problems and worrying about my future, I extend a warm thank you to my advisor, Vladimir Retakh. For a willingness to work through even the most boring of details if it would make me feel better, I extend a warm thank you to my advisor, Robert L. Wilson. For helpful mathematical discussions during my time at Rutgers, I would like to acknowledge Earl Taft, Jacob Towber, Kia Dalili, Sasa Radomirovic, Michael Richter, and the David Nacin Memorial Lecture Series—Nacin, Weingart, Schutzer. A most heartfelt thank you is extended to 326 Wayne ST, Maria, Kia, Saˇsa,Laura, and Ray. Without your steadying influence and constant comraderie, my time at Rut- gers may have been shorter, but certainly would have been darker. Thank you. Before there was Maria and 326 Wayne ST, there were others who supported me. -
GROMOV-WITTEN INVARIANTS on GRASSMANNIANS 1. Introduction
JOURNAL OF THE AMERICAN MATHEMATICAL SOCIETY Volume 16, Number 4, Pages 901{915 S 0894-0347(03)00429-6 Article electronically published on May 1, 2003 GROMOV-WITTEN INVARIANTS ON GRASSMANNIANS ANDERS SKOVSTED BUCH, ANDREW KRESCH, AND HARRY TAMVAKIS 1. Introduction The central theme of this paper is the following result: any three-point genus zero Gromov-Witten invariant on a Grassmannian X is equal to a classical intersec- tion number on a homogeneous space Y of the same Lie type. We prove this when X is a type A Grassmannian, and, in types B, C,andD,whenX is the Lagrangian or orthogonal Grassmannian parametrizing maximal isotropic subspaces in a com- plex vector space equipped with a non-degenerate skew-symmetric or symmetric form. The space Y depends on X and the degree of the Gromov-Witten invariant considered. For a type A Grassmannian, Y is a two-step flag variety, and in the other cases, Y is a sub-maximal isotropic Grassmannian. Our key identity for Gromov-Witten invariants is based on an explicit bijection between the set of rational maps counted by a Gromov-Witten invariant and the set of points in the intersection of three Schubert varieties in the homogeneous space Y . The proof of this result uses no moduli spaces of maps and requires only basic algebraic geometry. It was observed in [Bu1] that the intersection and linear span of the subspaces corresponding to points on a curve in a Grassmannian, called the kernel and span of the curve, have dimensions which are bounded below and above, respectively. -
Mathematisches Forschungsinstitut Oberwolfach Arithmetic Geometry
Mathematisches Forschungsinstitut Oberwolfach Report No. 38/2016 DOI: 10.4171/OWR/2016/38 Arithmetic Geometry Organised by Gerd Faltings, Bonn Johan de Jong, New York Peter Scholze, Bonn 7 August – 13 August 2016 Abstract. Arithmetic geometry is at the interface between algebraic geom- etry and number theory, and studies schemes over the ring of integers of number fields, or their p-adic completions. An emphasis of the workshop was on p-adic techniques, but various other aspects including Hodge theory, Arakelov theory and global questions were discussed. Mathematics Subject Classification (2010): 11G99. Introduction by the Organisers The workshop Arithmetic Geometry was well attended by over 50 participants from various backgrounds. It covered a wide range of topics in algebraic geometry and number theory, with some focus on p-adic questions. Using the theory of perfectoid spaces and related techniques, a number of results have been proved in recent years. At the conference, Caraiani, Gabber, Hansen and Liu reported on such results. In particular, Liu explained general p-adic versions of the Riemann–Hilbert and Simpson correspondences, and Caraiani reported on results on the torsion in the cohomology of Shimura varieties. This involved the geometry of the Hodge–Tate period map, which Hansen extended to a general Shimura variety, using the results reported by Liu. Moreover, Gabber proved degeneration of the Hodge spectral sequence for all proper smooth rigid spaces over nonarchimedean fields of characteristic 0, or even in families, by proving a spreading out result for proper rigid spaces to reduce to a recent result in p-adic Hodge theory. -
Algebraic D-Modules and Representation Theory Of
Contemporary Mathematics Volume 154, 1993 Algebraic -modules and Representation TheoryDof Semisimple Lie Groups Dragan Miliˇci´c Abstract. This expository paper represents an introduction to some aspects of the current research in representation theory of semisimple Lie groups. In particular, we discuss the theory of “localization” of modules over the envelop- ing algebra of a semisimple Lie algebra due to Alexander Beilinson and Joseph Bernstein [1], [2], and the work of Henryk Hecht, Wilfried Schmid, Joseph A. Wolf and the author on the localization of Harish-Chandra modules [7], [8], [13], [17], [18]. These results can be viewed as a vast generalization of the classical theorem of Armand Borel and Andr´e Weil on geometric realiza- tion of irreducible finite-dimensional representations of compact semisimple Lie groups [3]. 1. Introduction Let G0 be a connected semisimple Lie group with finite center. Fix a maximal compact subgroup K0 of G0. Let g be the complexified Lie algebra of G0 and k its subalgebra which is the complexified Lie algebra of K0. Denote by σ the corresponding Cartan involution, i.e., σ is the involution of g such that k is the set of its fixed points. Let K be the complexification of K0. The group K has a natural structure of a complex reductive algebraic group. Let π be an admissible representation of G0 of finite length. Then, the submod- ule V of all K0-finite vectors in this representation is a finitely generated module over the enveloping algebra (g) of g, and also a direct sum of finite-dimensional U irreducible representations of K0. -
The Duality Theorem on a Generalized Flag Variety
proceedings of the american mathematical society Volume 117, Number 3, March 1993 REMARKS ON LOCALIZATION AND STANDARD MODULES: THE DUALITY THEOREM ON A GENERALIZED FLAG VARIETY JEN-TSEH CHANG (Communicated by Jonathan M. Rosenberg) Abstract. An amplification of the duality theorem of Hecht, Milicic, Schmid, and Wolf is given in the setting of a generalized flag variety. As an application, we give a different proof of the reducibility theorem for principal series by Speh and Vogan. The purpose of this note is to sketch an amplification of the duality theo- rem of Hecht, Milicic, Schmid, and Wolf. The main theorem in [3] establishes a natural duality between Harish-Chandra modules constructed in two differ- ent ways: the ^-module construction on a flag variety (due to Beilinson and Bernstein) on the one hand and the cohomological induction from a Borel sub- algebra (due to Zuckerman) on the other hand. This also follows from a result of Bernstein, which gives a ^-module construction of Zuckerman's functors (for the construction, see [1]). In this note we introduce a similar Z)-module construction on a generalized flag variety. Following Hecht, Milicic, Schmid, and Wolf, this construction is then dual to the cohomological induction from a parabolic subalgebra (when containing a a-stable Levi factor). As an applica- tion of this result, we give a different proof of a result by Speh and Vogan [4], which asserts the irreducibility of certain standard Zuckerman modules. This introduction is followed by four sections. The first section recalls the notion of a generalized t.d.o. on a generalized flag variety introduced in [2], from which our construction stems. -
LIE GROUPS and ALGEBRAS NOTES Contents 1. Definitions 2
LIE GROUPS AND ALGEBRAS NOTES STANISLAV ATANASOV Contents 1. Definitions 2 1.1. Root systems, Weyl groups and Weyl chambers3 1.2. Cartan matrices and Dynkin diagrams4 1.3. Weights 5 1.4. Lie group and Lie algebra correspondence5 2. Basic results about Lie algebras7 2.1. General 7 2.2. Root system 7 2.3. Classification of semisimple Lie algebras8 3. Highest weight modules9 3.1. Universal enveloping algebra9 3.2. Weights and maximal vectors9 4. Compact Lie groups 10 4.1. Peter-Weyl theorem 10 4.2. Maximal tori 11 4.3. Symmetric spaces 11 4.4. Compact Lie algebras 12 4.5. Weyl's theorem 12 5. Semisimple Lie groups 13 5.1. Semisimple Lie algebras 13 5.2. Parabolic subalgebras. 14 5.3. Semisimple Lie groups 14 6. Reductive Lie groups 16 6.1. Reductive Lie algebras 16 6.2. Definition of reductive Lie group 16 6.3. Decompositions 18 6.4. The structure of M = ZK (a0) 18 6.5. Parabolic Subgroups 19 7. Functional analysis on Lie groups 21 7.1. Decomposition of the Haar measure 21 7.2. Reductive groups and parabolic subgroups 21 7.3. Weyl integration formula 22 8. Linear algebraic groups and their representation theory 23 8.1. Linear algebraic groups 23 8.2. Reductive and semisimple groups 24 8.3. Parabolic and Borel subgroups 25 8.4. Decompositions 27 Date: October, 2018. These notes compile results from multiple sources, mostly [1,2]. All mistakes are mine. 1 2 STANISLAV ATANASOV 1. Definitions Let g be a Lie algebra over algebraically closed field F of characteristic 0. -
Localization of Cohomologically Induced Modules to Partial Flag Varieties
LOCALIZATION OF COHOMOLOGICALLY INDUCED MODULES TO PARTIAL FLAG VARIETIES by Sarah Kitchen A dissertation submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Mathematics The University of Utah March 2010 Copyright c Sarah Kitchen 2010 All Rights Reserved THE UNIVERSITY OF UTAH GRADUATE SCHOOL SUPERVISORY COMMITTEE APPROVAL of a dissertation submitted by Sarah Kitchen This dissertation has been read by each member of the following supervisory committee and by majority vote has been found to be satisfactory. Chair: Dragan Miliˇci´c Aaron Bertram Peter Trapa Henryk Hecht Y.P. Lee THE UNIVERSITY OF UTAH GRADUATE SCHOOL FINAL READING APPROVAL To the Graduate Council of the University of Utah: I have read the dissertation of Sarah Kitchen in its final form and have found that (1) its format, citations, and bibliographic style are consistent and acceptable; (2) its illustrative materials including figures, tables, and charts are in place; and (3) the final manuscript is satisfactory to the Supervisory Committee and is ready for submission to The Graduate School. Date Dragan Miliˇci´c Chair, Supervisory Committee Approved for the Major Department Aaron Bertram Chair/Dean Approved for the Graduate Council Charles A. Wight Dean of The Graduate School ABSTRACT Abstract goes here. ?? CONTENTS ABSTRACT ...................................................... ii CHAPTERS 1. INTRODUCTION ............................................. 1 1.1 Cohomological Induction and Harish-Chandra Modules . 1 1.2 The Kazhdan-Lusztig Conjectures . 2 1.3 Localization of Harish-Chandra Modules . 3 1.4 The Beilinson-Ginzburg Equivariant Derived Category . 5 1.5 Equivariant Zuckerman Functors . -
Totally Positive Toeplitz Matrices and Quantum Cohomology of Partial Flag Varieties
JOURNAL OF THE AMERICAN MATHEMATICAL SOCIETY Volume 16, Number 2, Pages 363{392 S 0894-0347(02)00412-5 Article electronically published on November 29, 2002 TOTALLY POSITIVE TOEPLITZ MATRICES AND QUANTUM COHOMOLOGY OF PARTIAL FLAG VARIETIES KONSTANZE RIETSCH 1. Introduction A matrix is called totally nonnegative if all of its minors are nonnegative. Totally nonnegative infinite Toeplitz matrices were studied first in the 1950's. They are characterized in the following theorem conjectured by Schoenberg and proved by Edrei. Theorem 1.1 ([10]). The Toeplitz matrix ∞×1 1 a1 1 0 1 a2 a1 1 B . .. C B . a2 a1 . C B C A = B .. .. .. C B ad . C B C B .. .. C Bad+1 . a1 1 C B C B . C B . .. .. a a .. C B 2 1 C B . C B .. .. .. .. ..C B C is totally nonnegative@ precisely if its generating function is of theA form, 2 (1 + βit) 1+a1t + a2t + =exp(tα) ; ··· (1 γit) i Y2N − where α R 0 and β1 β2 0,γ1 γ2 0 with βi + γi < . 2 ≥ ≥ ≥···≥ ≥ ≥···≥ 1 This beautiful result has been reproved many times; see [32]P for anP overview. It may be thought of as giving a parameterization of the totally nonnegative Toeplitz matrices by ~ N N ~ (α;(βi)i; (~γi)i) R 0 R 0 R 0 i(βi +~γi) < ; f 2 ≥ × ≥ × ≥ j 1g i X2N where β~i = βi βi+1 andγ ~i = γi γi+1. − − Received by the editors December 10, 2001 and, in revised form, September 14, 2002. 2000 Mathematics Subject Classification. Primary 20G20, 15A48, 14N35, 14N15. -
Galois Cohomology and Homogeneous Varieties
GALOIS COHOMOLOGY IN DEGREE THREE AND HOMOGENEOUS VARIETIES∗ by Emmanuel Peyre Abstract. — The central result of this paper is the following generalization of a result of the author on products of Severi•Brauer varieties. Let G be a semi•simple linear algebraic group over a field k. Let V be a generalized flag variety under G. Then there exist finite extensions ki of k for 1 6 i 6 m, elements αi in Brki and a natural exact sequence m N (. α ) ki /k i k ∪ Ker H3(k,Q/Z(2)) H3(k(V ),Q/Z(2)) CH2(V ) 0. i∗ −−−−−−→ → → tors→ Mi=1 After giving a more explicit expression of the second morphism in a particular case, we apply this result to get classes in H3(Q,Q/Z), which are k•negligible for any field k of characteristic different from 2 which contains a fourth root of unity, for a group Q which is a central extension of an F2 vector space by another. Résumé. — Le résultat central de ce texte est la généralisation suivante d’un résultat de l’auteur sur les produits de variétés de Severi•Brauer. Soit G un groupe algébrique linéaire semi•simple sur un corps k. Soit V une variété de drapeaux généralisée sous G. Alors il existe des extensions finies ki de k pour 1 6 i 6 m, des éléments αi de Brki et une suite exacte naturelle m N (. α ) ki /k i k ∪ Ker H3(k,Q/Z(2)) H3(k(V ),Q/Z(2)) CH2(V ) 0. -
Contents 1 Root Systems
Stefan Dawydiak February 19, 2021 Marginalia about roots These notes are an attempt to maintain a overview collection of facts about and relationships between some situations in which root systems and root data appear. They also serve to track some common identifications and choices. The references include some helpful lecture notes with more examples. The author of these notes learned this material from courses taught by Zinovy Reichstein, Joel Kam- nitzer, James Arthur, and Florian Herzig, as well as many student talks, and lecture notes by Ivan Loseu. These notes are simply collected marginalia for those references. Any errors introduced, especially of viewpoint, are the author's own. The author of these notes would be grateful for their communication to [email protected]. Contents 1 Root systems 1 1.1 Root space decomposition . .2 1.2 Roots, coroots, and reflections . .3 1.2.1 Abstract root systems . .7 1.2.2 Coroots, fundamental weights and Cartan matrices . .7 1.2.3 Roots vs weights . .9 1.2.4 Roots at the group level . .9 1.3 The Weyl group . 10 1.3.1 Weyl Chambers . 11 1.3.2 The Weyl group as a subquotient for compact Lie groups . 13 1.3.3 The Weyl group as a subquotient for noncompact Lie groups . 13 2 Root data 16 2.1 Root data . 16 2.2 The Langlands dual group . 17 2.3 The flag variety . 18 2.3.1 Bruhat decomposition revisited . 18 2.3.2 Schubert cells . 19 3 Adelic groups 20 3.1 Weyl sets . 20 References 21 1 Root systems The following examples are taken mostly from [8] where they are stated without most of the calculations. -
Borel Subgroups and Flag Manifolds
Topics in Representation Theory: Borel Subgroups and Flag Manifolds 1 Borel and parabolic subalgebras We have seen that to pick a single irreducible out of the space of sections Γ(Lλ) we need to somehow impose the condition that elements of negative root spaces, acting from the right, give zero. To get a geometric interpretation of what this condition means, we need to invoke complex geometry. To even define the negative root space, we need to begin by complexifying the Lie algebra X gC = tC ⊕ gα α∈R and then making a choice of positive roots R+ ⊂ R X gC = tC ⊕ (gα + g−α) α∈R+ Note that the complexified tangent space to G/T at the identity coset is X TeT (G/T ) ⊗ C = gα α∈R and a choice of positive roots gives a choice of complex structure on TeT (G/T ), with the holomorphic, anti-holomorphic decomposition X X TeT (G/T ) ⊗ C = TeT (G/T ) ⊕ TeT (G/T ) = gα ⊕ g−α α∈R+ α∈R+ While g/t is not a Lie algebra, + X − X n = gα, and n = g−α α∈R+ α∈R+ are each Lie algebras, subalgebras of gC since [n+, n+] ⊂ n+ (and similarly for n−). This follows from [gα, gβ] ⊂ gα+β The Lie algebras n+ and n− are nilpotent Lie algebras, meaning 1 Definition 1 (Nilpotent Lie Algebra). A Lie algebra g is called nilpotent if, for some finite integer k, elements of g constructed by taking k commutators are zero. In other words [g, [g, [g, [g, ··· ]]]] = 0 where one is taking k commutators.