DOCSLIB.ORG

COTENSOR PRODUCTS of MODULES 1. Introduction Cotensor Product and Its Derived Functors Cotori Were Originally Introduced by Eile

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
COTENSOR PRODUCTS of MODULES 1. Introduction Cotensor Product and Its Derived Functors Cotori Were Originally Introduced by Eile TRANSACTIONS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 354, Number 6, Pages 2173{2185 S 0002-9947(02)02883-0 Article electronically published on February 1, 2002 COTENSOR PRODUCTS OF MODULES L. ABRAMS AND C. WEIBEL Abstract. Let C beacoalgebraoverafieldk and A its dual algebra. The category of C-comodules is equivalent to a category of A-modules. We use this to interpret the cotensor product MN of two comodules in terms of the appropriate Hochschild cohomology of the A-bimodule M ⊗ N,whenA is finite-dimensional, profinite, graded or differential-graded. The main applica- tions are to Galois cohomology, comodules over the Steenrod algebra, and the homology of induced fibrations. 1. Introduction Cotensor product and its derived functors Cotori were originally introduced by Eilenberg-Moore [EM] to calculate the homology of an induced fibration. These functors have been discussed in the literature, primarily in homotopy theory, but they have developed a reputation of being inaccessible. To counter this, we describe cotensor product and Cotor in terms of Hochschild cohomology, and provide a new class of examples to which they are relevant. Let C be a coalgebra over a field k and A its dual algebra. It is classical [Car] that one can identify comodules over C with a subcategory of modules over A. Thus comodule constructions may be translated into module constructions. In this paper, we interpret the cotensor product MN of two comodules as the vector space HomAe (A; M ⊗ N) of bimodule maps from A ⊗ ∗ to M N. We also interpret the right derived functors CotorC (M;N)intermsof the Hochschild cohomology of the A-bimodule M ⊗ N. Our results involve several apparently disparate areas of mathematics. n When C and A are finite-dimensional, we do indeed have CotorC (M;N)= Hn(A; M ⊗ N). This straightforward result is presented in Theorem 3.4. When C is infinite-dimensional, it is important to think of A as a topological algebra, as first suggested by Radford [Rad]. In this case, we prove in Theorem n n ⊗ 4.11 that CotorC (M;N)isthecontinuous Hochschild cohomology Hctn(A; M N), which we define in 4.6 below. This result has an interesting application to the Galois cohomology of a profinite group G. The profinite group ring k[G] is dual to the coalgebra C of locally constant ∗ functions from G to k, and the Galois cohomology Hctn(G; M)ofG with coefficients ∗ in a discrete G-module M is the same as CotorC (M;k); see Section 4 for details. ∗ When C is graded and each Cn is finite-dimensional, we prove that CotorC (M;N) ∗ is the graded Hochschild cohomology Hgr(M;N), at least when the graded modules Received by the editors April 18, 2000 and, in revised form, June 14, 2001. 2000 Mathematics Subject Classification. Primary 16E30; Secondary 16W30, 16E40. The second author was partially supported by NSF grants. c 2002 American Mathematical Society 2173 License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use 2174 L. ABRAMS AND C. WEIBEL M and N are bounded above (Theorem 5.6). Here there is extra bookkeeping for indices, but no topology is needed. ∗ For example, consider the Steenrod algebra A over Fp,andletX; Y denote topological spaces. Their homology groups are comodules over the dual A∗,and n n ∗ × we have CotorA∗ (H∗(X);H∗(Y )) = H (A ;H∗(X Y )); see Section 5. In Section 6 we extend our results to the differential graded case, and tie them in with the original formulation and application of Eilenberg and Moore [EM]. The special case when A is a Frobenius algebra (and therefore finite-dimensional) was previously considered by the first author in [Abr]. The construction there makes use of the special duality characterizing Frobenius algebras to produce a coalgebra structure on the dual of A and to translate the comodule constructions to module constructions. Proposition 3.3 here fills in a gap in the proof of [Abr, 4.6.1]. Notation. The symbol ⊗ denotes tensor product over k, and the term \algebra" always refers to a k-algebra. For any vector space V ,writeDV for the dual space Homk(V;k). When V is finite-dimensional there are natural isomorphisms ∼ ∼ D(V1 ⊗ V2) = (DV1) ⊗ (DV2)andV = D(DV ). For a k-algebra A, Ae denotes the enveloping algebra A ⊗ Aop.Thusamap e B1 ! B2 of A{A bimodules may be described as a map of left A -modules. We write HomAe (B1;B2) for the set of such maps. 2. Modules and comodules The results in this section are well known; they appear in [Car] and later on in [Sw]. Let k be a field and A a finite-dimensional algebra. It is elementary (see [Sw, 1.1.2]) that the dual of the structure maps k ! A and A ⊗ A ! A make the dual DA =Homk(A; k) into a co-associative coalgebra. If M is a left A-module, the formula ∆M (m)(a)=am defines a map ∼ ∆M : M ! Homk(A; M)=DA ⊗ M = M ⊗ DA : By [Car, 4-03] or [Sw, 2.1.2], this makes M into a right DA-comodule. Con- versely, if M is a right DA-comodule the map 1⊗∆ ∼ ev⊗1 A ⊗ M −→ A ⊗ (M ⊗ DA) = (A ⊗ DA) ⊗ M −→ M makes M intoaleftA-module by [Sw, 2.1.1]. As observed in loc. cit., these processes are inverse to each other, and module maps correspond to comodule maps by [Car] or [Sw, 2.1.3(e)]. We summarize this as follows. Theorem 2.1. Let A be a finite-dimensional algebra. Then there is an equivalence between the category of left A-modules and the category of right DA-comodules. ∼ A-mod = comod-DA Note in particular that the comodule structure map ∼ ∆A : A ! Homk(A; A) = Endk(A) is the right regular representation: ∆A(b)(a)=ab. In particular, the composition ∼ with k ! A yields the usual trace map τA : k ! A ⊗ DA = Endk(A). When M is finite-dimensional, we can use duality to give another interpretation of this equivalence. If M is any left A-module, Homk(M;k) is naturally a right A-module by the rule (fa)(m)=f(am). If M is finite-dimensional, the dual of the module structure map ρ: DM ⊗ A ! DM is a map Dρ: M ! M ⊗ DA; License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use COTENSOR PRODUCTS OF MODULES 2175 it is straightforward that Dρ makes M into a right DA-comodule. Elementary considerations show that this duality M 7! DM gives an equivalence between the category of finite-dimensional left A-modules and the category of finite-dimensional right DA-comodules. Lemma 2.2. When M is finite-dimensional, the structure map ∆M agrees with the duality map Dρ: M ! M ⊗ DA. i Proof. Let feig be a basis for A and fe g the dual basis of DA. By inspection, when M = A we have i i (Dρ)(1)(e ⊗ ej)=ρ(e ⊗ ej)(1) = δij ; P i so (Dρ)(1) = ei ⊗ e =∆A(1). Now given m 2 M,letfm : A ! M be the module homomorphism with fm(1) = m. By naturality of ρ and ∆, ∆M (m)=(fm ⊗ DA)∆A(1) = (fm ⊗ DA)(Dρ)(1) = Dρ(m): Note that even when M is not finite-dimensional, and hence Dρ might not be a comodule structure map, the map ∆M can still be expressed by ∆M (m)= (fm ⊗ DA)(Dρ)(1), where fm : A ! M is as in the proof of the lemma. Any construction on A-modules translates into a corresponding construction on DA-comodules. The contragradient representation is one example. Definition 2.3. Let C be a finite-dimensional coalgebra. If M is a right C- comodule, DM =Homk(M;k)istheleftC-comodule with structure map ∼ ∆=∆DM : DM ! DM ⊗ C = Hom(M;C) defined by the formula (∆f)(m)=(f ⊗ 1)(∆m). We leave it to the reader to check that this is the comodule structure obtained by regarding M as a left module over A = DC, and translating the right A-module structure on DM into a left DA-comodule structure. Remark 2.4. As an exercise, the reader might enjoy verifying that ∆DM is the dual of the map µM : A ⊗ M ! M. 3. Cotensor product and Cotor In this section, A will be a finite-dimensional k-algebra. If M is a left A-module, and N arightA-module, then the results of the last section allow us to view M as a right DA-comodule, and N as a left DA-comodule. As such, we can form the cotensor product MN = MDAN. Recall from [EM] [Rav] that MN is defined to be the kernel of the map φ = φM;N : M ⊗ N ! M ⊗ DA ⊗ N; φ(m ⊗ n)=∆1(m) ⊗ n − m ⊗ ∆2(n): Here ∆1 : M ! M ⊗ DA and ∆2 : N ! DA ⊗ N are the comodule structure maps. Proposition 3.1. If M and N are left and right A-modules, respectively, then there is a natural isomorphism ∼ MDAN = HomAe (A; M ⊗ N) License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use 2176 L. ABRAMS AND C. WEIBEL Proof. Note first that every bimodule map f : A ! M ⊗ N is determined by f(1) 2 ⊗ ⊗ ! ⊗ 7! M N,sothemapHomAe (A; M NP) M N defined by f f(1) is injective. To identify the image, consider ! = mi ⊗ ni. The assertion that ! = f(1) for some bimodule map f is equivalent to the condition X X (∗) a · mi ⊗ ni = mi ⊗ ni · a; for all a 2 A: Now, (∗) holds if and only if, for all a 2 A, X X (1M ⊗ a ⊗ 1N )( ∆1(mi) ⊗ ni)=(1M ⊗ a ⊗ 1N )( mi ⊗ ∆2(ni)); or, equivalently, if and only if X X ∆1(mi) ⊗ ni = mi ⊗ ∆2(ni); i.e., if and only if ! 2 MN.Thusf(1) 2 MN for all bimodule maps f,and each ! 2 MN defines a bimodule map f with f(1) = !: Remark 3.2.
Load more

Recommended publications
  • Cocommutative Hopf Algebras with Antipode by Moss Eisenberg Sweedler B.S., Massachusetts Institute of Technology SUBMITTED in PA
    J- Cocommutative Hopf Algebras with Antipode by Moss Eisenberg Sweedler B.S., Massachusetts Institute of Technology (1963) SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY August, 1965 Signature of Author . .-. .. Department of Mathematics, August 31, 1965 Certified by ....-.. Thesis Supervisqr Accepted by .......... 0................................... Chairman, Departmental Committee on Graduate Students v/I 2. Cocommutative Hopf Algebras 19 with Antipode by Moss Eisenberg Sweedler Submitted to the Department of Mathematics on August 31, 1965, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Abstract In the first chapter the preliminaries of the theory of Hopf algebras are presented. The notion and properties of the antipode are developed. An important filtration is induced in the Hopf algebra by its dual when the Hopf alge- bra is split. It is shown conilpotence and an algebraically closed field insure a Hopf algebra is split. The monoid of grouplike elements is studied. In the second chapter conditions for an algebra A -- which is a comodule for a Hopf algebra H --to be of the form A 'E B ® H (linear isomorphism) are given. The dual situation is studied. The graded Hopf algebra associated with a split Hopf algebra decomposes in the above manner. Chapter III contains the cohomology theory of a commutative algebra which is a module for a cocommutative Hopf algebra. There is extension theory and specialization to the situation the Hopf algebra is a group algebra. Chapter IV is dual to chapter III. Chapter V is devoted to coconnected cocommutative Hopf algebras, mostly in characteristic p > 0 .
    [Show full text]
  • Module and Comodule Categories - a Survey
    Module and Comodule Categories - a Survey Robert Wisbauer University of D¨usseldorf,Germany e-mail [email protected] Abstract The theory of modules over associative algebras and the theory of comodules for coassociative coalgebras were developed fairly indepen- dently during the last decades. In this survey we display an intimate connection between these areas by the notion of categories subgenerated by an object. After a review of the relevant techniques in categories of left modules, applications to the bimodule structure of algebras and comodule categories are sketched. 1. Module theory: Homological classification, the category σ[M], Morita equivalence, the functor ring, Morita dualities, decompositions, torsion theo- ries, trace functor. 2. Bimodule structure of an algebra: Multiplication algebra, Azumaya rings, biregular algebras, central closure of semiprime algebras. 3. Coalgebras and comodules: C-comodules and C∗-modules, σ-decom- position, rational functor, right semiperfect coalgebras, duality for comodules. B 4. Bialgebras and bimodules: The category B, coinvariants, B as B M projective generator in B, fundamental theorem for Hopf algebras, semiper- fect Hopf algebras. M 5. Comodule algebras: (A-H)-bimodules, smash product A#H∗, coin- H variants, A as progenerator in A . 6. Group actions and moduleM algebras: Group actions on algebras, A GA as a progenerator in σ[A GA], module algebras, smash product A#H, ∗ ∗ A#H A as a progenerator in σ[A#H A]. 1 1 Module theory In this section we recall mainly those results from module categories which are of interest for the applications to bimodules and comodules given in the subsequent sections.
    [Show full text]
  • Arxiv:Q-Alg/9509023V1 22 Sep 1995
    July 1993 Published in Advances in Hopf Algebras, Marcel Dekker Lec. Notes Pure and Applied Maths 158 (1994) 55-105. ALGEBRAS AND HOPF ALGEBRAS IN BRAIDED CATEGORIES1 SHAHN MAJID2 Department of Applied Mathematics & Theoretical Physics University of Cambridge, Cambridge CB3 9EW, U.K. ABSTRACT This is an introduction for algebraists to the theory of algebras and Hopf algebras in braided categories. Such objects generalise super-algebras and super-Hopf algebras, as well as colour-Lie algebras. Basic facts about braided categories C are recalled, the modules and comodules of Hopf algebras in such categories are studied, the notion of ‘braided-commutative’ or ‘braided-cocommutative’ Hopf algebras (braided groups) is reviewed and a fully diagrammatic proof of the reconstruction theorem for a braided group Aut (C) is given. The theory has important implications for the theory of quasitriangular Hopf algebras (quantum groups). It also includes important examples such as the degenerate Sklyanin algebra and the quantum plane. One of the main motivations of the theory of Hopf algebras is that they provide a gener- arXiv:q-alg/9509023v1 22 Sep 1995 alization of groups. Hopf algebras of functions on groups provide examples of commutative Hopf algebras, but it turns out that many group-theoretical constructions work just as well when the Hopf algebra is allowed to be non-commutative. This is the philosophy associated to some kind of non-commutative (or so-called quantum) algebraic geometry. In a Hopf algebra context one can say the same thing in a dual way: group algebras and enveloping algebras are cocommutative but many constructions are not tied to this.
    [Show full text]
  • Algebras and Cyclic Homology
    Homology of L∞-algebras and Cyclic Homology Masoud Khalkhali∗ A celebrated theorem of Loday and Quillen [LQ] and (independently) Tsygan [T] states that the Lie algebra homology of the Lie algebra of stable matrices over an associative algebra is canonically isomorphic, as a Hopf algebra, to the exterior power of the cyclic homology of the associative algebra. The main point of this paper is to lay the ground such that an extension of this theorem to the category of A∞-algebras becomes possible (theorem 3.1). The category of L∞- (respectively, A∞-) algebras extend the category of differential graded (DG) Lie (respectively, DG associative) algebras. These concepts are both due to J. Stasheff. See [S], [LS], and references therein, and also [HS] where an alternative approach to L∞-algebras is given. In [Kh], we proposed an approach to homological invariants of A∞- algebras (Hochschild, cyclic, periodic cyclic, etc.) based on the notion of X-complex due to Cuntz and Quillen [CQ]. It seems that it is now possible to extend most of the tools of noncommutative geometry of Connes [C] to the homotopical setting of A∞ and L∞-algebras. There is, however, a notable exception in that so far we don’t know how the K-theory of an A∞-algebra should be defined. 1 L∞ and A∞ algebras Let V be a vector space (not graded). Let ScV denote the cofree cocommutative counital coassociative coalgebra generated by V . Over fields of characteristic zero there are two c c ⊗n different constructions for S V that we recall now. First, let T V = M V be the cofree n≥0 counital coassociative coalgebra generated by V .
    [Show full text]
  • The Freedom of Yetter-Drinfeld Hopf Algebras
    Advances in Pure Mathematics, 2014, 4, 522-528 Published Online September 2014 in SciRes. http://www.scirp.org/journal/apm http://dx.doi.org/10.4236/apm.2014.49060 The Freedom of Yetter-Drinfeld Hopf Algebras Yanhua Wang School of Mathematics, Shanghai University of Finance and Economics, Shanghai, China Email: [email protected] Received 1 August 2014; revised 2 September 2014; accepted 13 September 2014 Copyright © 2014 by author and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/ Abstract In this paper, the fundamental theorem of Yetter-Drinfeld Hopf module is proved. As applications, the freedom of tensor and twisted tensor of two Yetter-Drinfeld Hopf algebras is given. Let A be a Yetter-Drinfeld Hopf algebra. It is proved that the category of A-bimodule is equivalent to the cat- egory of AA⊗ -twisted module. Keywords Hopf Algebra, Hopf Module, Yetter-Drinfeld Module, Yetter-Drinfeld Hopf Algebra 1. Introduction Let k be a field and A an algebra. A left A -module is a k -vector space V together with a k -linear map ⊗→ such that →=→ → and →=. The category of left -module is denoted by AV V abvabv( ) 1 vv A ∆ ε A M . Dually, let (C,,) be a coalgebra. A left C -comodule is a k -vector space V together with a k -linear map ρ :V→⊗ VC such that −10 a−−1⊗⊗ a 1 a 0= a − 1⊗ a 0⊗ a 0, ε aa−10= a. ∑∑( )12( ) ( ) ( ) ∑( ) C The category of left C -comodule is denoted by M .
    [Show full text]
  • On the Structure of Irreducible Yetter-Drinfeld Modules Over Quasi
    On the Structure of Irreducible Yetter-Drinfeld Modules over Quasi-Triangular Hopf Algebras Zhimin Liu∗ Shenglin Zhu †‡ Fudan University, Shanghai 200433, China Abstract Let (H, R) be a finite dimensional semisimple and cosemisimple quasi-triangular Hopf algebra over a field k. In this paper, we give the structure of irreducible objects of H the Yetter-Drinfeld module category H YD. Let HR be the Majid’s transmuted braided group of (H, R) , we show that HR is cosemisimple. As a coalgebra, let HR = D1 ⊕ ···⊕Dr be the sum of minimal H-adjoint-stable subcoalgebras. For each i (1 ≤ i ≤ r), we choose a minimal left coideal Wi of Di, and we can define the R-adjoint-stable algebra NWi of Wi. Using Ostrik’s theorem on characterizing module categories over H monoidal categories, we prove that V ∈ H YD is irreducible if and only if there exists an i (1 ≤ i ≤ r) and an irreducible right N -module U , such that V ∼ U ⊗ (H ⊗ W ). Wi i = i NWi i Our structure theorem generalizes the results of Dijkgraaf-Pasquier-Roche and Gould on Yetter-Drinfeld modules over finite group algebras. If k is an algebraically closed field of characteristic, we stress that the R-adjoint-stable algebra NWi is an al- gebra over which the dimension of each irreducible right module divides its dimension. KEYWORDS: Quasi-triangular Hopf algebra, Yetter-Drinfeld module, Transmuted braided group 2000 MATHEMATICS SUBJECT CLASSIFICATION: 16W30 1 Introduction arXiv:1811.05593v2 [math.RA] 17 Jun 2019 Yetter-Drinfeld modules over a bialgebra were introduced by Yetter [25] in 1990.
    [Show full text]
  • Appendices a Contramodules Over Coalgebras Over Fields
    Appendices A Contramodules over Coalgebras over Fields Let C be a coassociative coalgebra with counit over a field k.Itiswellknown (see [82, Theorems 2.1.3 and 2.2.1] or [68, 5.1.1]) that C is the union of its finite- dimensional subcoalgebras and any C-comodule is a union of finite-dimensional comodules over finite-dimensional subcoalgebras of C. The dual assertion for C-contramodules is not true: for the most common of noncosemisimple infinite- dimensional coalgebras C there exist C-contramodules P such that the intersection of the images of Homk(C/U, P)inP over all finite-dimensional subcoalgebras U ⊂ C is nonzero. A weaker statement holds, however: if the contraaction map Homk(C/U, P) −−→ P is surjective for every finite-dimensional subcoalgebra U of C,thenP = 0. Besides, even though adic filtrations of contramodules are not in general separated, they are always complete. Using the related techniques we show that any contraflat C-con- tramodule is projective, generalizing the well-known result that any flat module over a finite-dimensional algebra is projective [7]. A.1 Counterexamples A.1.1 Let C be the coalgebra for which the dual algebra C∗ is isomorphic to the algebra of formal power series k[[x]]. Then a C-contramodule P can be equivalently defined as a k-vector space endowed with the following operation of summation of n sequences of vectors with formal coefficients x : for any elements p0, p1, ... in P, ∞ n an element of P denoted by n=0 x pn is defined. This operation should satisfy the following equations: ∞ ∞ ∞ n n n x (apn + bqn)=a x pn + b x qn n=0 n=0 n=0 for a, b ∈ k, pn, qn ∈ P (linearity); ∞ n x pn = p0 when p1 = p2 = ···=0 n=0 (counity); and ∞ ∞ ∞ i j n x x pij = x pij i=0 j=0 n=0 i+j=n for any pij ∈ P, i, j =0, 1,..
    [Show full text]
  • Indecomposable Coalgebras, Simple Comodules, And
    proceedings of the american mathematical society Volume 123, Number 8, August 1995 INDECOMPOSABLE COALGEBRAS,SIMPLE COMODULES, AND POINTED HOPF ALGEBRAS SUSAN MONTGOMERY (Communicated by Lance W. Small) Abstract. We prove that every coalgebra C is a direct sum of coalgebras in such a way that the summands correspond to the connected components of the Ext quiver of the simple comodules of C . This result is used to prove that every pointed Hopf algebra is a crossed product of a group over the indecomposable component of the identity element. Introduction A basic structure theorem for cocommutative coalgebras asserts that any such coalgebra C is a direct sum of its irreducible components; as a consequence, it can be shown that any pointed cocommutative Hopf algebra is a skew group ring of the group G of group-like elements of H over the irreducible component of the identity element. These results were proved independently by Cartier and Gabriel and by Kostant in the early 1960's; see [Di, SI]. This paper is concerned with versions of these results for arbitrary coalgebras and for arbitrary (pointed) Hopf algebras. In fact much is already known about the coalgebra problem. In 1975 Ka- plansky [K] showed that any coalgebra C is (uniquely) a direct sum of inde- composable ones; moreover when C is cocommutative, the indecomposable components are irreducible. In 1978 Shudo and Miyamoto [ShM] defined an equivalence relation on the set of simple subcoalgebras of C and showed that the equivalence classes correspond to the indecomposable summands. A weaker version of this equivalence relation was studied recently in [XF].
    [Show full text]
  • Arxiv:1911.12847V1 [Math.QA] 28 Nov 2019 Omto Ruod Ekbagba Ekhp Algebra
    ALGEBRAIC STRUCTURES IN COMODULE CATEGORIES OVER WEAK BIALGEBRAS CHELSEA WALTON, ELIZABETH WICKS, AND ROBERT WON Abstract. For a bialgebra L coacting on a k-algebra A, a classical result states that A is a right L-comodule algebra if and only if A is an algebra in the monoidal category ML of right L-comodules; the former notion is formulaic while the latter is categorical. We generalize this result to the setting of weak bialgebras H. The category MH admits a monoidal structure by work of Nill and B¨ohm-Caenepeel-Janssen, but the algebras in MH are not canonically k-algebras. Nevertheless, we prove that there is an isomorphism between the category of right H-comodule algebras and the category of algebras in MH . We also recall and introduce the formulaic notion of H coacting on a k-coalgebra and on a Frobenius k-algebra, respectively, and prove analogous category isomorphism results. Our work is inspired by the physical applications of Frobenius algebras in tensor cate- gories and by symmetries of algebras with a base algebra larger than the ground field (e.g. path algebras). We produce examples of the latter by constructing a monoidal functor from a certain corepresentation category of a bialgebra L to the corepresenta- tion category of a weak bialgebra built from L (a “quantum transformation groupoid”), thereby creating weak quantum symmetries from ordinary quantum symmetries. Contents 1. Introduction 2 2. Preliminaries on weak bialgebras 5 2.1. Algebraic structures over a field 5 2.2. Terminology and properties 6 2.3. Modules and comodules 7 2.4.
    [Show full text]
  • HOPF ALGEBRAS 1. Algebras and Coalgebras 1.1. Algebras. We Start by Formulating a Familiar Definition in a Possibly Unfa- Miliar
    HOPF ALGEBRAS 1. Algebras and coalgebras 1.1. Algebras. We start by formulating a familiar de¯nition in a possibly unfa- miliar way: De¯nition. A k-algebra is a k-vector space with two linear maps m : A ­k A ! A and u : k ! A satisfying the following conditions hold. These respectively encode associativity and the unit element: (a) the diagram m­id A ­ A ­ A / A ­ A id­m m ² m ² A ­ A / A is commutative; and (b) the diagram A ­ A u: dII u­id uu IIid­u uu II uu II uu I m k ­ AJ A ­ k JJ tt JJ tt JJ tt s JJ tt s J$ ² ztt A (where s denotes scalar multiplication) is commutative. We get 1A as u(1k) via the second diagram. 1.2. Coalgebras. To de¯ne these, we reverse the arrows in (7.1): De¯nition. A k-coalgebra is a k-vector space, C, with two k-linear maps, ¢ (co- multiplication or coproduct) and " (counit), with ¢ : C ! C ­ C and " : C ! k; such that the following conditions, respectively called the coassociativity and the counit axioms, hold: 1 2 HOPF ALGEBRAS a) the diagram ¢o ­id C ­ CO ­ CC ­O C id­¢ ¢ C ­ C o C ¢ commutes; b) the diagram C ­ C t O JJ "­id tt JJid­" tt JJ tt JJ ztt J$ k ­ CCJd ¢ : ­ k JJ tt JJ1­ ­1 tt JJ tt JJ tt J tt C commutes, where 1 ­ is the map x 7! 1 ­ x. 1.3. Morphisms. De¯nition. Let A and B be k-algebras.
    [Show full text]
  • Graded Twisting of Comodule Algebras and Module Categories
    GRADED TWISTING OF COMODULE ALGEBRAS AND MODULE CATEGORIES JULIEN BICHON, SERGEY NESHVEYEV, AND MAKOTO YAMASHITA Abstract. Continuing our previous work on graded twisting of Hopf algebras and monoidal categories, we introduce a graded twisting construction for equivariant comodule algebras and module categories. As an example we study actions of quantum subgroups of G ⊂ SL−1(2) G on K−1[x; y] and show that in most cases the corresponding invariant rings K−1[x; y] are 0 invariant rings K[x; y]G for the action of a classical subgroup G0 ⊂ SL(2). As another example we study Poisson boundaries of graded twisted categories and show that under the assumption of weak amenability they are graded twistings of the Poisson boundaries. Introduction Let A, B be Hopf algebras, and assume that B is a 2-cocycle twisting of A. Then we have a monoidal equivalence between the categories of left comodules F : Comod(A) '⊗ Comod(B) It is a simple observation that F induces an equivalence between the respective categories of comodule algebras, and that if R is an A-comodule algebra, we have an isomorphism between the fixed point algebras coBF(R) ' coAR In other words, the invariant theory of A determines that of B. One of the main goals of this paper is to give a version of this principle when B is a graded twisting of A [BNY16]. In this case there is also an equivalence Comod(A) ' Comod(B), but it is not monoidal in general, so we cannot expect a correspondence between the categories of comodule algebras.
    [Show full text]
  • Koszulity of Cohomology= $ K (\Pi, 1) $-Ness+ Quasi-Formality
    KOSZULITY OF COHOMOLOGY = K(π, 1)-NESS + QUASI-FORMALITY LEONID POSITSELSKI Abstract. This paper is a greatly expanded version of [37, Section 9.11]. A series of definitions and results illustrating the thesis in the title (where quasi-formality means vanishing of a certain kind of Massey multiplications in the cohomology) is presented. In particular, we include a categorical interpretation of the “Koszulity implies K(π, 1)” claim, discuss the differences between two versions of Massey op- erations, and apply the derived nonhomogeneous Koszul duality theory in order to deduce the main theorem. In the end we demonstrate a counterexample providing a negative answer to a question of Hopkins and Wickelgren about formality of the cochain DG-algebras of absolute Galois groups, thus showing that quasi-formality cannot be strengthened to formality in the title assertion. Contents Introduction 1 1. Koszulity Implies K(π, 1)-ness 5 2. Koszulity Implies Quasi-Formality 10 3. Noncommutative (Rational) Homotopy Theory 18 4. K(π, 1)-ness + Quasi-Formality Imply Koszulity 23 5. Self-Consistency of Nonhomogeneous Quadratic Relations 26 6. Koszulity Does Not Imply Formality 33 References 37 arXiv:1507.04691v8 [math.KT] 21 Apr 2017 Introduction A quadratic algebra is an associative algebra defined by homogeneous quadratic relations. In other words, a positively graded algebra A = k ⊕ A1 ⊕ A2 ⊕··· over a field k is called quadratic if it is generated by its first-degree component A1 with relations in degree 2. A positively graded associative algebra A is called Koszul [41, A 4, 32] if one has Torij(k,k) = 0 for all i =6 j, where the first grading i on the Tor spaces is the usual homological grading and the second grading j, called the internal grading, is induced by the grading of A.
    [Show full text]