DOCSLIB.ORG

Representations and Cohomology of Categories

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Representations and Cohomology of Categories Representations and Cohomology of Categories Peter Webb What is a representation of a category? Representations and Cohomology of Category cohomology and the Schur Categories multiplier Xu's counterexample Peter Webb The orbit category and Alperin's weight conjecture University of Minnesota Concluding remarks April 10, 2010 Representations Outline and Cohomology of Categories Peter Webb What is a representation of a What is a representation of a category? category? Category cohomology and the Schur Category cohomology and the Schur multiplier multiplier Xu's counterexample The orbit category Xu's counterexample and Alperin's weight conjecture Concluding The orbit category and Alperin's weight conjecture remarks Concluding remarks Representations Theme and Cohomology of Categories Peter Webb What is a representation of a category? Category cohomology and the Schur multiplier Xu's Representations of categories are remarkably like counterexample representations of groups! The orbit category and Alperin's weight conjecture Concluding remarks Representations Categories and Cohomology of Categories Peter Webb Let C be a small category. What is a representation of a Examples: category? Category I a group cohomology and the Schur I a poset multiplier Xu's I the free category associated to a quiver. The objects counterexample are the vertices of the quiver, the morphisms are all The orbit category and Alperin's possible composable strings of the arrows. weight conjecture Concluding The theory of representations of the above examples is well remarks developed and we do not expect to get more information about them from this general theory. We are more interested in other categories, such as the orbit category associated to a family of subgroups of a group, or the categories which arise with p-local finite groups. Representations Representations and Cohomology of Categories Peter Webb What is a representation of a Let R be a commutative ring with 1. A representation of a category? Category category C over R is a functor M : C! R-mod. cohomology and the Schur multiplier Straightforward example: Xu's C is the category with five morphisms • − • −! •. counterexample The orbit category A representation is a diagram of modules B − A −! C. and Alperin's We may be interested in weight conjecture Concluding I the direct limit of this diagram: the pushout; remarks I is this operation exact? I Etc. Representations and Cohomology of Categories Peter Webb What is a representation of a category? Category cohomology and the Schur multiplier A representation of a category is a diagram of modules. Xu's counterexample The orbit category and Alperin's weight conjecture Concluding remarks Representations Well-studied examples of representations and Cohomology of Categories Peter Webb What is a representation of a I When C is a group we get homomorphism category? C! EndR (V ). Category cohomology and I When C is a poset we get a module for the incidence the Schur multiplier algebra. Xu's counterexample I When C is the free category associated to a quiver we The orbit category get a representation of the quiver. and Alperin's weight conjecture When C = • − • −! • its path algebra is I Concluding remarks 0∗ ∗ ∗1 @0 ∗ 0A 0 0 ∗ Representations Further examples and Cohomology of Categories Peter Webb What is a representation of a category? Category cohomology and I C = finite dimensional vector spaces over some field. the Schur multiplier We get generic representation theory. Xu's counterexample I C = finite sets with bijective morphisms. We get The orbit category species. and Alperin's weight conjecture Various constructions in topology and the cohomology I Concluding of groups: homotopy colimits, the Quillen category. remarks Representations Category Algebra and Cohomology of Categories Peter Webb What is a representation of a category? The category algebra RC is the free R-module with the Category cohomology and morphisms of C as a basis. We define the product of these the Schur basis elements to be composition if possible, zero otherwise. multiplier Xu's Examples: counterexample The orbit category I When C is a group we get the group algebra. and Alperin's weight conjecture When C is a poset we get the incidence algebra. I Concluding remarks I When C is the free category associated to a quiver we get the path algebra of the quiver. Representations Equivalence of representations and modules and Cohomology of Categories Peter Webb Theorem (B. Mitchell) What is a representation of a Representations are `the same' as RC-modules, if C has category? Category finitely many objects. cohomology and the Schur multiplier Example: Xu's counterexample I When C is a group, representations are the same as The orbit category and Alperin's modules for the group algebra. weight conjecture When C is the free category associated to a quiver, Concluding I remarks representations are the same as modules for the path algebra. Under this correspondence a representation M corresponds L to an RC-module x2ObC M(x). Natural transformations of functors correspond to module homomorphisms. Representations Constant functors and Cohomology of Categories Peter Webb What is a representation of a category? Category cohomology and For any R-module A we define the constant functor the Schur A : C! R-mod to be A(x) = A on objects x and multiplier Xu's A(α) = idA on morphisms α. counterexample Taking A to be R itself we get the constant functor R. The orbit category and Alperin's Example: weight conjecture Concluding I When C is a group we get the trivial module R. remarks Representations Theme and Cohomology of Categories Peter Webb What is a representation of a category? Category cohomology and the Schur multiplier Xu's Representations of categories are remarkably like counterexample representations of groups! The orbit category and Alperin's weight conjecture Concluding remarks Representations Category cohomology and Cohomology of Categories Peter Webb What is a representation of a Theorem (Roos, Gabriel-Zisman) category? ∗ ∼ ∗ ExtRC(R; R) = H (jCj; R) where jCj is the nerve of C. Category cohomology and the Schur multiplier We define H∗(C; R) to be the cohomology groups in the last Xu's theorem. This is the cohomology of C. counterexample More generally, for any representation M of C we put The orbit category and Alperin's ∗ ∗ weight conjecture H (C; M) := ExtRC(R; M). Concluding remarks Example: I When C is a (discrete) group the nerve is the classifying space BC and the algebraically computed cohomology is isomorphic to the cohomology of BC. Representations Category extensions: Definition 1 and Cohomology of Categories Peter Webb What is a representation of a category? Extension definition EZ: Category cohomology and An extension of a category C is a diagram of categories and the Schur functors multiplier Xu's K!E!C counterexample The orbit category which behaves like a group extension and Alperin's weight conjecture Concluding 1 ! K ! E ! G ! 1 remarks (i.e. a short exact sequence of groups). Representations Category extensions: Definition 2 and Cohomology of Categories Peter Webb An extension of a category C (in the sense of Hoff) is a What is a representation of a diagram of categories and functors category? Category i p cohomology and K −!E −!C the Schur multiplier Xu's satisfying counterexample The orbit category 1. K, E and C all have the same objects, i and p are the and Alperin's identity on objects, i is injective on morphisms, and p is weight conjecture Concluding surjective on morphisms; remarks 2. whenever f and g are morphisms in E then p(f ) = p(g) if and only if there exists a morphism m 2 K for which f = i(m)g. In that case, the morphism m is required to be unique. Representations Extension properties and Cohomology of Categories Peter Webb i p Given an extension K −!E −!C it follows (not obviously) What is a representation of a that category? Category I all morphisms in K are endomorphisms, and are cohomology and the Schur invertible, multiplier I we get a functor E! Groups, x 7! EndK(x). Xu's counterexample The orbit category If all the groups End (x) are abelian and Alperin's K weight conjecture I we get a functor C! AbelianGroups Concluding remarks i.e. a representation of C, which we denote K. Compare: for a group extension 1 ! K ! E ! G ! 1 there is a conjugation action of E on the normal subgroup K. When K is abelian it becomes a representation of G. Representations Second cohomology parametrizes extensions and Cohomology of Categories Peter Webb What is a representation of a category? Category cohomology and the Schur multiplier Theorem Xu's When all the groups EndK(x) are abelian, equivalence counterexample classes of extensions K!E!C biject with elements of The orbit category and Alperin's H2(C; K). weight conjecture Concluding remarks Representations Other interpretations of cohomology and Cohomology of Categories Peter Webb What is a representation of a category? Category cohomology and the Schur 1 0 multiplier There are known interpretations of H , H , H0, H1 which Xu's generalize to categories the familiar results for groups. counterexample A generalization to categories of the group-theoretic The orbit category and Alperin's interpretation of H2 has not previously been observed. weight conjecture Concluding remarks Representations Schur multiplier basics and Cohomology of Categories Peter Webb What is a The Schur multiplier of a category C is defined to be representation of a category? H (C; ) = TorRC( ; ). This generalizes the definition for 2 Z 2 Z Z Category groups. cohomology and the Schur Theorem multiplier 0 Xu's Let G be a group for which G=G is free abelian. There is counterexample universal central extension 1 ! K ! E ! G ! 1 with The orbit category 0 and Alperin's K ⊆ E , unique up to isomorphism.
Load more

Recommended publications
  • Nonablian Cocycles and Their Σ-Model Qfts
    Nonablian cocycles and their σ-model QFTs December 31, 2008 Abstract Nonabelian cohomology can be regarded as a generalization of group cohomology to the case where both the group itself as well as the coefficient object are allowed to be generalized to 1-groupoids or even to general 1-categories. Cocycles in nonabelian cohomology in particular represent higher principal bundles (gerbes) { possibly equivariant, possibly with connection { as well as the corresponding associated higher vector bundles. We propose, expanding on considerations in [13, 34, 5], a systematic 1-functorial formalization of the σ-model quantum field theory associated with a given nonabelian cocycle regarded as the background field for a brane coupled to it. We define propagation in these σ-model QFTs and recover central aspects of groupoidification [1, 2] of linear algebra. In a series of examples we show how this formalization reproduces familiar structures in σ-models with finite target spaces such as Dijkgraaf-Witten theory and the Yetter model. The generalization to σ-models with smooth target spaces is developed elsewhere [24]. 1 Contents 1 Introduction 3 2 1-Categories and Homotopy Theory 4 2.1 Shapes for 1-cells . 5 2.2 !-Categories . 5 2.3 !-Groupoids . 7 2.4 Cosimplicial !-categories . 7 2.5 Monoidal biclosed structure on !Categories ............................ 7 2.6 Model structure on !Categories ................................... 9 3 Nonabelian cohomology and higher vector bundles 9 3.1 Principal !-bundles . 10 3.2 Associated !-bundles . 11 3.3 Sections and homotopies . 12 4 Quantization of !-bundles to σ-models 15 4.1 σ-Models . 16 4.2 Branes and bibranes .
    [Show full text]
  • THE MCM-APPROXIMATION of the TRIVIAL MODULE OVER a CATEGORY ALGEBRA3 Unfactorizable Morphism
    THE MCM-APPROXIMATION OF THE TRIVIAL MODULE OVER A CATEGORY ALGEBRA REN WANG Abstract. For a finite free EI category, we construct an explicit module over its category algebra. If in addition the category is projective over the ground field, the constructed module is Gorenstein-projective and is a maximal Cohen- Macaulay approximation of the trivial module. We give conditions on when the trivial module is Gorenstein-projective. 1. Introduction Let k be a field and C be a finite EI category. Here, the EI condition means that all endomorphisms in C are isomorphisms. In particular, HomC (x, x) = AutC (x) is a finite group for each object x. Denote by kAutC (x) the group algebra. Recall that a finite EI category C is projective over k if each kAutC (y)-kAutC (x)-bimodule kHomC (x, y) is projective on both sides; see [9, Definition 4.2]. The concept of a finite free EI category is introduced in [6, 7]. Let C be a finite α free EI category. For any morphism x → y in C , set V (α) to be the set of objects ′ ′′ α α w such that there are factorizations x → w → y of α with α′′ a non-isomorphism. ′′ For any w ∈ V (α), we set tw(α) = α ◦ ( g), which is an element in g∈AutPC (w) kHomC (w,y). The freeness of C implies that the element tw(α) is independent of the choice of α′′. Denote by k-mod the category of finite dimensional k-vector spaces. We identify covariant functors from C to k-mod with left modules over the category algebra.
    [Show full text]
  • Pathlike Co/Bialgebras and Their Antipodes with Applications to Bi- and Hopf Algebras Appearing in Topology, Number Theory and Physics
    PATHLIKE CO/BIALGEBRAS AND THEIR ANTIPODES WITH APPLICATIONS TO BI- AND HOPF ALGEBRAS APPEARING IN TOPOLOGY, NUMBER THEORY AND PHYSICS. RALPH M. KAUFMANN AND YANG MO Dedicated to Professor Dirk Kreimer on the occasion of his 60th birthday Abstract. We develop an algebraic theory of flavored and pathlike co-, bi- and Hopf algebras. This is the right framework in which to discuss antipodes for bialgebras naturally appearing in topology, number theory and physics. In particular, we can precisely give conditions the invert- ibility of characters needed for renormalization in the formulation of Connes and Kreimer and in the canonical examples these conditions are met. To construct the antipodes directly, we discuss formal localization constructions and quantum deformations. These allow to construct and naturally explain the appearance Brown style coactions. Using previous results, we can tie all the relevant coalgebras to categorical constructions and the bialgebra structures to Feynman categories. Introduction Although the use of Hopf algebras has a long history, the seminal pa- per [CK98] led to a turbocharged development for their use which has pene- trated deeply into mathematical physics, number theory and also topology, their original realm|see [AFS08] for the early beginnings. The important realization in [CK98, CK00, CK01] was that the renormalization procedure for quantum field theory can be based on a character theory for Hopf al- gebras via a so-called Birkhoff factorization and convolution inverses. The relevant Hopf algebras are those of trees with a coproduct given by a sum over so{called admissible cuts |with the factors of the coproduct being the arXiv:2104.08895v1 [math.QA] 18 Apr 2021 left over stump and the collection of cut off branches| and Hopf algebras of graphs in which the factors of the summands of the coproduct are given by a subgraph and the quotient graph.
    [Show full text]
  • Quiver Generalized Weyl Algebras, Skew Category Algebras and Diskew Polynomial Rings
    Math.Comput.Sci. (2017) 11:253–268 DOI 10.1007/s11786-017-0313-5 Mathematics in Computer Science Quiver Generalized Weyl Algebras, Skew Category Algebras and Diskew Polynomial Rings V. V. B av u l a Received: 7 December 2016 / Revised: 3 March 2017 / Accepted: 11 March 2017 / Published online: 28 April 2017 © The Author(s) 2017. This article is an open access publication Abstract The aim of the paper is to introduce new large classes of algebras—quiver generalized Weyl algebras, skew category algebras, diskew polynomial rings and skew semi-Laurent polynomial rings. Keywords Skew category algebra · Quiver generalized Weyl algebra · Diskew polynomial ring · Generalized Weyl algebra · Skew double quiver algebra · Double quiver groupoid Mathematics Subject Classification 16D30 · 16P40 · 16D25 · 16P50 · 16S85 1 Introduction In this paper, K is a commutative ring with 1, algebra means a K -algebra. In general, it is not assumed that a K -algebra has an identity element. Module means a left module. Missing definitions can be found in [11]. The aim of the paper is to introduce new large classes of algebras—quiver generalized Weyl algebras, skew category algebras, diskew polynomial rings, skew semi-Laurent polynomial rings and the simplex generalized Weyl algebras. 2 Skew Category Algebras The aim of this section is to introduce skew category algebras; to consider new interesting examples (skew tree rings, skew semi-Laurent polynomial rings, etc); to give criteria for a skew category algebra to be a left/right Noetherian algebra (Theorem 2.2, Proposition 2.3). In Sect. 3, skew category algebras are used to define the quiver generalized Weyl algebras.
    [Show full text]
  • Monoids in Representation Theory, Markov Chains, Combinatorics and Operator Algebras
    Monoids in Representation Theory, Markov Chains, Combinatorics and Operator Algebras Benjamin Steinberg, City College of New York Australian Mathematical Society Meeting December 2019 Group representation theory and Markov chains • Group representation theory is sometimes a valuable tool for analyzing Markov chains. Group representation theory and Markov chains • Group representation theory is sometimes a valuable tool for analyzing Markov chains. • The method (at least for nonabelian groups) was perhaps first popularized in a paper of Diaconis and Shahshahani from 1981. Group representation theory and Markov chains • Group representation theory is sometimes a valuable tool for analyzing Markov chains. • The method (at least for nonabelian groups) was perhaps first popularized in a paper of Diaconis and Shahshahani from 1981. • They analyzed shuffling a deck of cards by randomly transposing cards. Group representation theory and Markov chains • Group representation theory is sometimes a valuable tool for analyzing Markov chains. • The method (at least for nonabelian groups) was perhaps first popularized in a paper of Diaconis and Shahshahani from 1981. • They analyzed shuffling a deck of cards by randomly transposing cards. • Because the transpositions form a conjugacy class, the eigenvalues and convergence time of the walk can be understood via characters of the symmetric group. Group representation theory and Markov chains • Group representation theory is sometimes a valuable tool for analyzing Markov chains. • The method (at least for nonabelian groups) was perhaps first popularized in a paper of Diaconis and Shahshahani from 1981. • They analyzed shuffling a deck of cards by randomly transposing cards. • Because the transpositions form a conjugacy class, the eigenvalues and convergence time of the walk can be understood via characters of the symmetric group.
    [Show full text]
  • Adjoint Associativity: an Invitation to Algebra in -Categories 1 JOSEPHLIPMAN
    Commutative Algebra and Noncommutative Algebraic Geometry, II MSRI Publications Volume 68, 2015 Adjoint associativity: an invitation to algebra in -categories 1 JOSEPH LIPMAN There appeared not long ago a reduction formula for derived Hochschild cohomology, that has been useful, for example, in the study of Gorenstein maps and of rigidity with respect to semidualizing complexes. The formula involves the relative dualizing complex of a ring homomorphism, so brings out a connection between Hochschild homology and Grothendieck duality. The proof, somewhat ad hoc, uses homotopical considerations via a number of noncanonical projective and injective resolutions of differential graded objects. Recent efforts aim at more intrinsic approaches, hopefully upgradable to “higher” contexts — like bimodules over algebras in -categories. This 1 would lead to wider applicability, for example to ring spectra; and the methods might be globalizable, revealing some homotopical generalizations of aspects of Grothendieck duality. (The original formula has a geometric version, proved by completely different methods coming from duality theory.) A first step is to extend Hom-Tensor adjunction — adjoint associativity — to the -category 1 setting. Introduction 266 1. Motivation: reduction of Hochschild (co)homology 266 2. Enter homotopy 268 3. Adjoint associativity 269 4. -categories 270 1 5. The homotopy category of an -category 273 1 6. Mapping spaces; equivalences 276 7. Colimits 279 8. Adjoint functors 280 9. Algebra objects in monoidal -categories 282 1 This expository article is an elaboration of a colloquium talk given April 17, 2013 at the Mathemat- ical Sciences Research Institute in Berkeley, under the Commutative Algebra program supported by the National Science Foundation.
    [Show full text]
  • Operator Algebras in Rigid C*-Tensor Categories Arxiv:1611.04620V2
    Operator algebras in rigid C*-tensor categories Corey Jones and David Penneys December 5, 2016 Abstract In this article, we define operator algebras internal to a rigid C*-tensor category C. A C*/W*-algebra object in C is an algebra object A in ind-C whose category of free modules FreeModC(A) is a C-module C*/W*-category respectively. When C = Hilbf:d:, the category of finite dimensional Hilbert spaces, we recover the usual notions of operator algebras. We generalize basic representation theoretic results, such as the Gelfand-Naimark and von Neumann bicommutant theorems, along with the GNS construction. We define the notion of completely positive maps between C*-algebra objects in C and prove the analog of the Stinespring dilation theorem. As an application, we discuss approximation and rigidity properties, including amenability, the Haagerup property, and property (T) for a connected W*-algebra M in C. Our definitions simultaneously unify the definitions of analytic properties for discrete quantum groups and rigid C*-tensor categories. Contents 1 Introduction2 2 Background7 2.1 Linear, dagger, and C*-categories . .8 2.2 Involutions on tensor categories . .9 2.3 Rigid C*-tensor categories . 10 arXiv:1611.04620v2 [math.OA] 1 Dec 2016 2.4 The category Vec(C)............................... 13 2.5 Graphical calculus for the Yoneda embedding . 15 2.6 The category Hilb(C)............................... 17 3 ∗-Algebras in Vec(C) 20 3.1 Algebras and module categories . 20 3.2 Equivalence between algebras and cyclic module categories . 22 3.3 ∗-Algebra objects and dagger module categories . 25 3.4 C* and W*-algebra objects .
    [Show full text]
  • Arxiv:1911.00818V2 [Math.CT] 12 Jul 2021 Oc Eerhlbrtr,Teus Oenet Rcrei M Carnegie Or Shou Government, Express and U.S
    A PRACTICAL TYPE THEORY FOR SYMMETRIC MONOIDAL CATEGORIES MICHAEL SHULMAN Abstract. We give a natural-deduction-style type theory for symmetric monoidal cat- egories whose judgmental structure directly represents morphisms with tensor products in their codomain as well as their domain. The syntax is inspired by Sweedler notation for coalgebras, with variables associated to types in the domain and terms associated to types in the codomain, allowing types to be treated informally like “sets with elements” subject to global linearity-like restrictions. We illustrate the usefulness of this type the- ory with various applications to duality, traces, Frobenius monoids, and (weak) Hopf monoids. Contents 1 Introduction 1 2 Props 9 3 On the admissibility of structural rules 11 4 The type theory for free props 14 5 Constructing free props from type theory 24 6 Presentations of props 29 7 Examples 31 1. Introduction 1.1. Type theories for monoidal categories. Type theories are a powerful tool for reasoning about categorical structures. This is best-known in the case of the internal language of a topos, which is a higher-order intuitionistic logic. But there are also weaker type theories that correspond to less highly-structured categories, such as regular logic for arXiv:1911.00818v2 [math.CT] 12 Jul 2021 regular categories, simply typed λ-calculus for cartesian closed categories, typed algebraic theories for categories with finite products, and so on (a good overview can be found in [Joh02, Part D]). This material is based on research sponsored by The United States Air Force Research Laboratory under agreement number FA9550-15-1-0053.
    [Show full text]
  • Arxiv:1406.4204V3 [Math.QA] 10 Mar 2018 C Linear Category Corepresenting C-Balanced Right-Exact Bilinear Functors out of M × N
    THE BALANCED TENSOR PRODUCT OF MODULE CATEGORIES CHRISTOPHER L. DOUGLAS, CHRISTOPHER SCHOMMER-PRIES, AND NOAH SNYDER Abstract. The balanced tensor product M ⊗A N of two modules over an algebra A is the vector space corepresenting A-balanced bilinear maps out of the product M ×N. The balanced tensor product MC N of two module categories over a monoidal linear category C is the linear category corepresenting C-balanced right-exact bilinear functors out of the product category M × N . We show that the balanced tensor product can be realized as a category of bimodule objects in C, provided the monoidal linear category is finite and rigid. Tensor categories are a higher dimensional analogue of algebras. Just as modules and bimod- ules play a key role in the theory of algebras, the analogous notions of module categories and bimodule categories play a key role in the study of tensor categories (as pioneered by Ostrik [Ost03]). One of the key constructions in the theory of modules and bimodules is the relative tensor product M ⊗A N. As first recognized by Tambara [Tam01], a similarly important role is played by the balanced tensor product of module categories over a monoidal category M C N (for example, see [M¨ug03, ENO10, JL09, GS12b, GS12a, DNO13, FSV13]). For C = Vect, this agrees with Deligne's [Del90] tensor product K L of finite linear categories. Etingof, Nikshych, and Ostrik [ENO10] established the existence of a balanced tensor product M C N of finite semisimple module categories over a fusion category, and Davydov and Nikshych [DN13, x2.7] outlined how to generalize this construction to module categories over a finite tensor category.1 We give a new construction of the balanced tensor product over a finite tensor category as a category of bimodule objects.
    [Show full text]
  • THE BOUNDED DERIVED CATEGORY of a POSET 3 Broader Application Than This Paper
    THE BOUNDED DERIVED CATEGORY OF A POSET KOSMAS DIVERIS, MARJU PURIN, AND PETER WEBB Abstract. We introduce a new combinatorial condition on a subin- terval of a poset P (a clamped subinterval) that allows us to relate the Auslander-Reiten quiver of the bounded derived category of P to that of the subinterval. Applications include the determination of when a poset is fractionally Calabi-Yau and the computation of the Auslander-Reiten quivers of both the bounded derived category and of the module category. 1. Introduction We study representations of a finite poset P over a field k. By this we mean representations of the incidence algebra kP; they may also be identified as functors from P to the category of vector spaces over k. Such representations have been an object of study since at least the pioneering work of Mitchell [11, 12]. A notable result was the deter- mination of the posets of finite representation type by Loupias [10]. More recent work has centered around the abstract properties of these representations, including the question of when the bounded derived category of an incidence algebra is fractionally Calabi-Yau [3, 7]. Ex- amples show that such questions do not seem to have a particularly straightforward answer in terms of the combinatorics of the poset. Our main goal in this paper is to introduce a technique that allows us to describe the representation theory of a wide class of posets exactly by arXiv:1709.03227v1 [math.RT] 11 Sep 2017 such combinatorial means. We develop a new method, which we call clamping, to describe the Auslander-Reiten quiver components of kP in both the module cate- gory and the derived category.
    [Show full text]
  • Arxiv:2010.13598V2 [Math.CT] 19 Mar 2021 Theory
    AFFINIZATION OF MONOIDAL CATEGORIES YOUSSEF MOUSAAID AND ALISTAIR SAVAGE Abstract. We define the affinization of an arbitrary monoidal category C, corresponding to the category of C-diagrams on the cylinder. We also give an alternative characterization in terms of adjoining dot generators to C. The affinization formalizes and unifies many constructions appearing in the literature. In particular, we describe a large number of examples coming from Hecke-type algebras, braids, tangles, and knot invariants. When C is rigid, its affinization is isomorphic to its horizontal trace, although the two definitions look quite different. In general, the affinization and the horizontal trace are not isomorphic. 1. Introduction The goal of the current paper is to formalize and unify the concept of affinization in certain areas of category theory and representation theory. In particular, we are interested in the following uses of the term affine: Topological/diagrammatic: The term affine is often used to refer to the topological or diagram- matic• setting of a torus, annulus, or cylinder. For example, one often encounters the term affine braids to refer to braids on a cylinder. More generally, the term affine is often used to describe either monoidal categories drawn in terms of string diagrams on a cylinder or annulus, or where strings are allowed to carry dots with various properties. The description in terms of dots is often ad hoc. For example, the dots are sometimes equal to their own mates and sometimes they are not. Algebraic: In the context of Hecke-type algebras, the term affine refers to the introduction of a (Laurent)• polynomial part of the algebra.
    [Show full text]
  • Quivers of Monoids with Basic Algebras
    COMPOSITIO MATHEMATICA Quivers of monoids with basic algebras Stuart Margolis and Benjamin Steinberg Compositio Math. 148 (2012), 1516{1560. doi:10.1112/S0010437X1200022X FOUNDATION COMPOSITIO MATHEMATICA Compositio Math. 148 (2012) 1516{1560 doi:10.1112/S0010437X1200022X Quivers of monoids with basic algebras Stuart Margolis and Benjamin Steinberg Abstract We compute the quiver of any finite monoid that has a basic algebra over an algebraically closed field of characteristic zero. More generally, we reduce the computation of the quiver over a splitting field of a class of monoids that we term rectangular monoids (in the semigroup theory literature the class is known as DO) to representation-theoretic computations for group algebras of maximal subgroups. Hence in good characteristic for the maximal subgroups, this gives an essentially complete computation. Since groups are examples of rectangular monoids, we cannot hope to do better than this. For the subclass of R-trivial monoids, we also provide a semigroup-theoretic description of the projective indecomposable modules and compute the Cartan matrix. 1. Introduction Whereas the representation theory of finite groups has played a central role in that theory and its applications for more than a century, the same cannot be said for the representation theory of finite semigroups. While the basic parts of representation theory of finite semigroups were developed in the 1950s by Clifford, Munn and Ponizovksy, [CP61, ch. 5], there were not ready- made applications of the theory, both internally in semigroup theory and in applications of that theory to other parts of mathematics and science. Thus, in a paper in 1971, [McA71], the only application mentioned of the theory was Rhodes's use of it to compute the Krohn{Rhodes complexity of a completely regular semigroup [KR68, Rho69].
    [Show full text]