DOCSLIB.ORG

Introduction to Derived Categories the first Lecture Will Be About the General Theory of Derived Categories

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Introduction to Derived Categories the first Lecture Will Be About the General Theory of Derived Categories Outline Here is an outline of the mini-course. Introduction to Derived Categories The first lecture will be about the general theory of derived categories. 1. The Homotopy Category Mini-Course at MSRI, Jan-Feb 2013 2. The Derived Category 3. Derived Functors 4. Resolutions Amnon Yekutieli 5. DG Algebras (new section; change of numbering below) Department of Mathematics The second lecture will be on more specialized topics, leaning towards Ben Gurion University noncommutative algebraic geometry. 5. Commutative Dualizing Complexes Notes available at http://www.math.bgu.ac.il/~amyekut/lectures 6. Noncommutative Dualizing Complexes Updated notes , 29 Jan 2013 7. Tilting Complexes and Derived Morita Theory 8. Rigid Dualizing Complexes Due to the time constraint I had to leave out some important topics (such as DG algebras). Amnon Yekutieli (BGU) Derived Categories 1 / 65 Amnon Yekutieli (BGU) Derived Categories 2 / 65 1. The Homotopy Category 1. The Homotopy Category A homomorphism of complexes φ : M → N is a commutative diagram 1. The Homotopy Category −1 0 − dM dM Suppose M is an abelian category . · · · / M 1 / M 0 / M 1 / · · · (1.1) The main examples for us are these: φ−1 φ0 φ1 ◮ M Mod / −1 / 0 / 1 / A is a ring, and = A, the category of left A-modules. · · · N −1 N 0 N · · · dN dN ◮ (X, A) is a ringed space, and M = Mod A, the category of sheaves of left A-modules. in M. C M M A complex in M is a diagram Let us denote by ( ) the category of complexes in . −1 0 It is again an abelian category; but it is also a differential graded (DG) dM dM M = · · · → M −1 −−−→ M 0 −−−→ M 1 → ··· category , as we now explain. M i+1! i in such that dM ◦ dM = 0 . Amnon Yekutieli (BGU) Derived Categories 3 / 65 Amnon Yekutieli (BGU) Derived Categories 4 / 65 1. The Homotopy Category 1. The Homotopy Category Given M, N ∈ C(M) we let Note that the abelian structure of C(M) can be recovered from the DG structure as follows: i j j+i Hom M(M, N ) := Hom M(M , N ) 0 ∈Z Hom C M (M, N ) = Z Hom M(M, N ) , jY ( ) the set of 0-cocycles. ! and i Indeed, for φ : M → N of degree 0 the condition d( φ) = 0 is equivalent Hom M(M, N ) := Hom M(M, N ) . ∈Z to the commutativity of the diagram (1.1). Mi For φ ∈ Hom M(M, N )i we let i d( φ) := d N ◦ φ − (−1) φ ◦ dM . In this way Hom M(M, N ) becomes a complex of abelian groups, i.e. a DG Z-module. Amnon Yekutieli (BGU) Derived Categories 5 / 65 Amnon Yekutieli (BGU) Derived Categories 6 / 65 1. The Homotopy Category 1. The Homotopy Category Next we define the homotopy category K(M). Suppose K is an additive category, with an automorphism T called the translation (or shift, or suspension). Its objects are the complexes in M (same as C(M)), and A triangle in K is a diagram of morphisms of this sort: 0 Hom K(M)(M, N ) = H Hom M(M, N ) . α β γ ! L −→ M −→ N −→ T (L). In other words, these are homotopy classes of homomorphisms The name comes from the alternative typesetting φ : M → N in C(M). N a γ β There is an additive functor C(M) → K(M), which is the identity on ~ objects and surjective on morphisms. L α / M The additive category K(M) is no longer abelian – it is a triangulated A triangulated category structure on K is a set of triangles called category . Let me explain what this means. distinguished triangles , satisfying a list of axioms (that are not so important for us). Details can be found in the references [Ye5], [Sc], [Ha], [We], [KS1], [Ne2] or [LH]. Amnon Yekutieli (BGU) Derived Categories 7 / 65 Amnon Yekutieli (BGU) Derived Categories 8 / 65 1. The Homotopy Category 1. The Homotopy Category The translation T of the category K(M) is defined as follows. A triangle in K(M) is distinguished if it is isomorphic, as a diagram in K M i i+1 ( ), to the triangle On objects we take T (M) := M and dT (M) := −dM . On morphisms i i+1 it is T (φ) := φ . L −→ α M −→ cone( α) −→ T (L) Given a homomorphism α : L → M in C(M), its cone is the complex for some homomorphism α : L → M in C(M). T (L) cone( α) := T (L) ⊕ M = A calculation shows that K(M) is indeed triangulated (i.e. the axioms M that I did not specify are satisfied). M Z with differential (in matrix notation) For M ∈ K( ) and i ∈ we will write i T (d L) 0 M[i] := T (M), d := . " T (α) d M # the i-th translation of M. The relation between distinguished triangles and exact sequences will There are canonical homomorphisms M → cone( α) and be mentioned later. cone( α) → T (L) in C(M). Amnon Yekutieli (BGU) Derived Categories 9 / 65 Amnon Yekutieli (BGU) Derived Categories 10 / 65 1. The Homotopy Category 2. The Derived Category Suppose K and K′ are triangulated categories. A triangulated functor F : K → K′ is an additive functor that commutes with the translations, 2. The Derived Category and sends distinguished triangles to distinguished triangles. As before M is an abelian category. M M′ Example 1.2. Let F : → be an additive functor (not Given a complex M ∈ C(M), we can consider its cohomologies necessarily exact) between abelian categories. Hi(M) := ker(d i )/ im(d i−1) ∈ M . Extend F to a functor M M C C M C M′ (F ) : ( ) → ( ) Since the cohomologies are homotopy-invariant, we get additive functors in the obvious way, namely Hi : K(M) → M . C(F )( M)i := F (M i) i K M for a complex M = {M }i∈Z. A morphism ψ : M → N in ( ) is called a quasi-isomorphism if Hi(ψ) are isomorphisms for all i. The functor C(F ) respects homotopies, so we get an additive functor Let us denote by S(M) the set of all quasi-isomorphisms in K(M). K(F ) : K(M) → K(M′). This is a triangulated functor. Amnon Yekutieli (BGU) Derived Categories 11 / 65 Amnon Yekutieli (BGU) Derived Categories 12 / 65 2. The Derived Category 2. The Derived Category M M Clearly S( ) is a multiplicatively closed set, i.e. the composition of two Every morphism χ : M → N in K( )S(M) can be written as quasi-isomorphisms is a quasi-isomorphism. −1 −1 χ = Q(φ1) ◦ Q(ψ1 ) = Q(ψ2 ) ◦ Q(φ2) A calculation shows that S(M) is a left and right denominator set (as in K M S M ring theory). for some φi ∈ ( ) and ψi ∈ ( ). M The category K(M)S M inherits a triangulated structure from K(M), It follows that the Ore localization K( )S(M) exists. This is an additive ( ) category, with object set and the localization functor Q is triangulated. M M There is a universal property: given a triangulated functor Ob( K( )S(M)) = Ob( K( )) . F : K(M) → E There is a functor to a triangulated category E, such that F (ψ) is an isomorphism for K M K M Q : ( ) → ( )S(M) every ψ ∈ S(M), there exists a unique triangulated functor called the localization functor, which is the identity on objects. M E FS(M) : K( )S(M) → such that FS(M) ◦ Q = F. Amnon Yekutieli (BGU) Derived Categories 13 / 65 Amnon Yekutieli (BGU) Derived Categories 14 / 65 2. The Derived Category 2. The Derived Category It turns out that the abelian structure of M can be recovered from this Definition 2.1. The derived category of the abelian category M is the embedding. triangulated category D(M) := K(M)S M . ( ) Proposition 2.3. Consider a sequence α β The derived category was introduced by Grothendieck and Verdier 0 → L −→ M −→ N → 0 around 1960. The first published material is the book “Residues and M Duality” [Ha] from 1966, written by Hartshorne following notes by in . Grothendieck. This sequence is exact iff there is a morphism γ : N → L[1] in D(M) D M 0 D M such that Let ( ) be the full subcategory of ( ) consisting of the complexes α β γ whose cohomology is concentrated in degree 0. L −→ M −→ N −→ L[1] is a distinguished triangle. Proposition 2.2. The obvious functor M → D(M)0 is an equivalence. This allows us to view M as an additive subcategory of D(M). Amnon Yekutieli (BGU) Derived Categories 15 / 65 Amnon Yekutieli (BGU) Derived Categories 16 / 65 3. Derived Functors 3. Derived Functors Definition 3.1. A right derived functor of F is a triangulated functor 3. Derived Functors D M E As before M is an abelian category. Recall the localization functor RF : ( ) → , Q : K(M) → D(M). together with a morphism It is a triangulated functor, which is the indentity on objects, and η : F → RF ◦ Q inverts quasi-isomorphisms. of triangulated functors K(M) → E, Suppose E is some triangulated category, and F : K(M) → E a satisfying this condition: triangulated functor. We now introduce the right and left derived functors of F . These are (∗) The pair (R F, η ) is initial among all such pairs. triangulated functors RF, LF : D(M) → E Being initial means that if (G, η ′) is another such pair, then there is a unique morphism of triangulated functors θ : R F → G s.t. η′ = θ ◦ η.
Load more

Recommended publications
  • The Homotopy Category Is a Homotopy Category
    Vol. XXIII, 19~2 435 The homotopy category is a homotopy category By A~NE S~o~ In [4] Quillen defines the concept of a category o/models /or a homotopy theory (a model category for short). A model category is a category K together with three distingxtished classes of morphisms in K: F ("fibrations"), C ("cofibrations"), and W ("weak equivalences"). These classes are required to satisfy axioms M0--M5 of [4]. A closed model category is a model category satisfying the extra axiom M6 (see [4] for the statement of the axioms M0--M6). To each model category K one can associate a category Ho K called the homotopy category of K. Essentially, HoK is obtained by turning the morphisms in W into isomorphisms. It is shown in [4] that the category of topological spaces is a closed model category ff one puts F---- (Serre fibrations) and IV = (weak homotopy equivalences}, and takes C to be the class of all maps having a certain lifting property. From an aesthetical point of view, however, it would be nicer to work with ordinary (Hurewicz) fibrations, cofibrations and homotopy equivalences. The corresponding homotopy category would then be the ordinary homotopy category of topological spaces, i.e. the objects would be all topological spaces and the morphisms would be all homotopy classes of continuous maps. In the first section of this paper we prove that this is indeed feasible, and in the last section we consider the case of spaces with base points. 1. The model category structure of Top. Let Top be the category of topolo~cal spaces and continuous maps.
    [Show full text]
  • Faithful Linear-Categorical Mapping Class Group Action 3
    A FAITHFUL LINEAR-CATEGORICAL ACTION OF THE MAPPING CLASS GROUP OF A SURFACE WITH BOUNDARY ROBERT LIPSHITZ, PETER S. OZSVÁTH, AND DYLAN P. THURSTON Abstract. We show that the action of the mapping class group on bordered Floer homol- ogy in the second to extremal spinc-structure is faithful. This paper is designed partly as an introduction to the subject, and much of it should be readable without a background in Floer homology. Contents 1. Introduction1 1.1. Acknowledgements.3 2. The algebras4 2.1. Arc diagrams4 2.2. The algebra B(Z) 4 2.3. The algebra C(Z) 7 3. The bimodules 11 3.1. Diagrams for elements of the mapping class group 11 3.2. Type D modules 13 3.3. Type A modules 16 3.4. Practical computations 18 3.5. Equivalence of the two actions 22 4. Faithfulness of the action 22 5. Finite generation 24 6. Further questions 26 References 26 1. Introduction arXiv:1012.1032v2 [math.GT] 11 Feb 2014 Two long-standing, and apparently unrelated, questions in low-dimensional topology are whether the mapping class group of a surface is linear and whether the Jones polynomial detects the unknot. In 2010, Kronheimer-Mrowka gave an affirmative answer to a categorified version of the second question: they showed that Khovanov homology, a categorification of the Jones polynomial, does detect the unknot [KM11]. (Previously, Grigsby and Wehrli had shown that any nontrivially-colored Khovanov homology detects the unknot [GW10].) 2000 Mathematics Subject Classification. Primary: 57M60; Secondary: 57R58. Key words and phrases. Mapping class group, Heegaard Floer homology, categorical group actions.
    [Show full text]
  • Homotopical Categories: from Model Categories to ( ,)-Categories ∞
    HOMOTOPICAL CATEGORIES: FROM MODEL CATEGORIES TO ( ;1)-CATEGORIES 1 EMILY RIEHL Abstract. This chapter, written for Stable categories and structured ring spectra, edited by Andrew J. Blumberg, Teena Gerhardt, and Michael A. Hill, surveys the history of homotopical categories, from Gabriel and Zisman’s categories of frac- tions to Quillen’s model categories, through Dwyer and Kan’s simplicial localiza- tions and culminating in ( ;1)-categories, first introduced through concrete mod- 1 els and later re-conceptualized in a model-independent framework. This reader is not presumed to have prior acquaintance with any of these concepts. Suggested exercises are included to fertilize intuitions and copious references point to exter- nal sources with more details. A running theme of homotopy limits and colimits is included to explain the kinds of problems homotopical categories are designed to solve as well as technical approaches to these problems. Contents 1. The history of homotopical categories 2 2. Categories of fractions and localization 5 2.1. The Gabriel–Zisman category of fractions 5 3. Model category presentations of homotopical categories 7 3.1. Model category structures via weak factorization systems 8 3.2. On functoriality of factorizations 12 3.3. The homotopy relation on arrows 13 3.4. The homotopy category of a model category 17 3.5. Quillen’s model structure on simplicial sets 19 4. Derived functors between model categories 20 4.1. Derived functors and equivalence of homotopy theories 21 4.2. Quillen functors 24 4.3. Derived composites and derived adjunctions 25 4.4. Monoidal and enriched model categories 27 4.5.
    [Show full text]
  • Rational Homotopy Theory: a Brief Introduction
    Contemporary Mathematics Rational Homotopy Theory: A Brief Introduction Kathryn Hess Abstract. These notes contain a brief introduction to rational homotopy theory: its model category foundations, the Sullivan model and interactions with the theory of local commutative rings. Introduction This overview of rational homotopy theory consists of an extended version of lecture notes from a minicourse based primarily on the encyclopedic text [18] of F´elix, Halperin and Thomas. With only three hours to devote to such a broad and rich subject, it was difficult to choose among the numerous possible topics to present. Based on the subjects covered in the first week of this summer school, I decided that the goal of this course should be to establish carefully the founda- tions of rational homotopy theory, then to treat more superficially one of its most important tools, the Sullivan model. Finally, I provided a brief summary of the ex- tremely fruitful interactions between rational homotopy theory and local algebra, in the spirit of the summer school theme “Interactions between Homotopy Theory and Algebra.” I hoped to motivate the students to delve more deeply into the subject themselves, while providing them with a solid enough background to do so with relative ease. As these lecture notes do not constitute a history of rational homotopy theory, I have chosen to refer the reader to [18], instead of to the original papers, for the proofs of almost all of the results cited, at least in Sections 1 and 2. The reader interested in proper attributions will find them in [18] or [24]. The author would like to thank Luchezar Avramov and Srikanth Iyengar, as well as the anonymous referee, for their helpful comments on an earlier version of this article.
    [Show full text]
  • Lecture Notes on Simplicial Homotopy Theory
    Lectures on Homotopy Theory The links below are to pdf files, which comprise my lecture notes for a first course on Homotopy Theory. I last gave this course at the University of Western Ontario during the Winter term of 2018. The course material is widely applicable, in fields including Topology, Geometry, Number Theory, Mathematical Pysics, and some forms of data analysis. This collection of files is the basic source material for the course, and this page is an outline of the course contents. In practice, some of this is elective - I usually don't get much beyond proving the Hurewicz Theorem in classroom lectures. Also, despite the titles, each of the files covers much more material than one can usually present in a single lecture. More detail on topics covered here can be found in the Goerss-Jardine book Simplicial Homotopy Theory, which appears in the References. It would be quite helpful for a student to have a background in basic Algebraic Topology and/or Homological Algebra prior to working through this course. J.F. Jardine Office: Middlesex College 118 Phone: 519-661-2111 x86512 E-mail: [email protected] Homotopy theories Lecture 01: Homological algebra Section 1: Chain complexes Section 2: Ordinary chain complexes Section 3: Closed model categories Lecture 02: Spaces Section 4: Spaces and homotopy groups Section 5: Serre fibrations and a model structure for spaces Lecture 03: Homotopical algebra Section 6: Example: Chain homotopy Section 7: Homotopical algebra Section 8: The homotopy category Lecture 04: Simplicial sets Section 9:
    [Show full text]
  • A Primer on Homotopy Colimits
    A PRIMER ON HOMOTOPY COLIMITS DANIEL DUGGER Contents 1. Introduction2 Part 1. Getting started 4 2. First examples4 3. Simplicial spaces9 4. Construction of homotopy colimits 16 5. Homotopy limits and some useful adjunctions 21 6. Changing the indexing category 25 7. A few examples 29 Part 2. A closer look 30 8. Brief review of model categories 31 9. The derived functor perspective 34 10. More on changing the indexing category 40 11. The two-sided bar construction 44 12. Function spaces and the two-sided cobar construction 49 Part 3. The homotopy theory of diagrams 52 13. Model structures on diagram categories 53 14. Cofibrant diagrams 60 15. Diagrams in the homotopy category 66 16. Homotopy coherent diagrams 69 Part 4. Other useful tools 76 17. Homology and cohomology of categories 77 18. Spectral sequences for holims and hocolims 85 19. Homotopy limits and colimits in other model categories 90 20. Various results concerning simplicial objects 94 Part 5. Examples 96 21. Homotopy initial and terminal functors 96 22. Homotopical decompositions of spaces 103 23. A survey of other applications 108 Appendix A. The simplicial cone construction 108 References 108 1 2 DANIEL DUGGER 1. Introduction This is an expository paper on homotopy colimits and homotopy limits. These are constructions which should arguably be in the toolkit of every modern algebraic topologist, yet there does not seem to be a place in the literature where a graduate student can easily read about them. Certainly there are many fine sources: [BK], [DwS], [H], [HV], [V1], [V2], [CS], [S], among others.
    [Show full text]
  • The Classification of Triangulated Subcategories 3
    Compositio Mathematica 105: 1±27, 1997. 1 c 1997 Kluwer Academic Publishers. Printed in the Netherlands. The classi®cation of triangulated subcategories R. W. THOMASON ? CNRS URA212, U.F.R. de Mathematiques, Universite Paris VII, 75251 Paris CEDEX 05, France email: thomason@@frmap711.mathp7.jussieu.fr Received 2 August 1994; accepted in ®nal form 30 May 1995 Key words: triangulated category, Grothendieck group, Hopkins±Neeman classi®cation Mathematics Subject Classi®cations (1991): 18E30, 18F30. 1. Introduction The ®rst main result of this paper is a bijective correspondence between the strictly full triangulated subcategories dense in a given triangulated category and the sub- groups of its Grothendieck group (Thm. 2.1). Since every strictly full triangulated subcategory is dense in a uniquely determined thick triangulated subcategory, this result re®nes any known classi®cation of thick subcategories to a classi®cation of all strictly full triangulated ones. For example, one can thus re®ne the famous clas- si®cation of the thick subcategories of the ®nite stable homotopy category given by the work of Devinatz±Hopkins±Smith ([Ho], [DHS], [HS] Thm. 7, [Ra] 3.4.3), which is responsible for most of the recent advances in stable homotopy theory. One can likewise re®ne the analogous classi®cation given by Hopkins and Neeman (R ) ([Ho] Sect. 4, [Ne] 1.5) of the thick subcategories of D parf, the chain homotopy category of bounded complexes of ®nitely generated projective R -modules, where R is a commutative noetherian ring. The second main result is a generalization of this classi®cation result of Hop- kins and Neeman to schemes, and in particular to non-noetherian rings.
    [Show full text]
  • Fulltekst (Pdf)
    Stable phenomena for some automorphism groups in topology Erik Lindell ©Erik Lindell, Stockholm, 2021 Address: Erik Lindell, Matematiska institutionen, Stockholms universitet, 106 91 Stockholm E-mail address: [email protected] ISBN 978-91-7797-991-3 Printed by Eprint AB 2021 Distributor: Department of Mathematics, Stockholm University Abstract This licentiate thesis consists of two papers about topics related to representation stability for different automorphisms groups of topological spaces and manifolds. In Paper I, we study the rational homology groups of Torelli groups of smooth, compact and orientable surfaces. The Torelli group of a smooth surface is the group of isotopy classes of orientation preserving diffeomor- phisms that act trivially on the first homology group of the surface. In the paper, we study a certain class of stable homology classes, i.e. classes that exist for sufficiently large genus, and explicitly describe the image of these classes under a higher degree version of the Johnson homomorphism, as a representation of the symplectic group. This gives a lower bound on the dimension of the stable homology of the group, as well as providing some further evidence that these homology groups satisfy representation stability for symplectic groups, in the sense of Church and Farb. In Paper II, we study pointed homotopy automorphisms of iterated wedge sums of spaces as well as boundary relative homotopy automor- phisms of iterated connected sums of manifolds with a disk removed. We prove that the rational homotopy groups of these, for simply connected CW-complexes and closed manifolds respectively, satisfy representation stability for symmetric groups, in the sense of Church and Farb.
    [Show full text]
  • Homotopy Type Theory: a New Connection Between Logic, Category Theory and Topology
    Homotopy type theory: a new connection between logic, category theory and topology Andr´eJoyal UQAM` Category Theory Seminar, CUNY, October 26, 2018 Classical connections between logic, algebra and topology I Boolean algebras I Heyting algebras I Cylindric algebras I Categorical doctrines I Cartesian closed categories and λ-calculus I Autonomous categories and linear logic Topos theoretic connections: I Frames and locales I Elementary toposes and intuitionistic set theory I Grothendieck toposes and geometric logic I Realisability toposes Axiomatic Homotopy Theory J.H.C. Whitehead (1950): The ultimate aim of algebraic homotopy is to construct a purely algebraic theory, which is equivalent to homotopy theory in the same sort of way that analytic is equivalent to pure projective geometry. Traditional axiomatic systems in homotopy theory: I Triangulated categories [Verdier 1963]; I Homotopical algebra [Quillen 1967]; I Derivators [Grothendieck 1984] New axiomatic systems: I Higher toposes [Rezk, Lurie, ....]; I Homotopy type theory [Voevodsky, Awodey & Warren,....]; I Cubical type theory [Coquand & collaborators]. Voevodsky's Univalent Foundation Program Voevodsky's univalence principle is to type theory what the induction principle is to Peano arithmetic. A univalent type theory (UTT) is obtained by adding the univalence principle to Martin-L¨oftype theory (MLTT). The goal of Voevodsky's Univalent Foundation Program is to I give constructive mathematics a new foundation; I apply type theory to homotopy theory; I develop proof assistants based in UTT. But many basic questions remain to be solved. Goals of my talk To describe the connection between type theory, category theory and topology by using the notion of tribe. Theorem (Gambino & Garner, Shulman) The syntaxic category of type theory is a tribe.
    [Show full text]
  • Realizing Diagrams in the Homotopy Category by Means of Diagrams of Simplicial Sets1 W
    PROCEEDINGS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 91, Number 3, July 1984 REALIZING DIAGRAMS IN THE HOMOTOPY CATEGORY BY MEANS OF DIAGRAMS OF SIMPLICIAL SETS1 W. G. DWYER AND D. M. KAN ABSTRACT. Given a small category D, we show that a D-diagram X in the homotopy category can be realized by a D-diagram of simplicial sets iff a certain simplicial set rX is nonempty. Moreover, this simplicial set rX can be expressed as the homotopy inverse limit of simplicial sets whose homtopy types are quite well understood. There is also an associated obstruction theory. In the special case that D is a group (i.e. D has only one object and all its maps are invertible) these results reduce to the ones of G. Cooke. 1. Introduction. 1.1. Summary. Let D be a small category. The aim of this note then is to obtain necessary and sufficient conditions in order that a D-diagram X in the homotopy category can be realized by means of a D-diagram of simplicial sets. This is done by constructing a simplicial set rX, which is nonempty iff X can be so realized. To get a hold on the homotopy type of rX, one notes that rX is the homotopy fibre of a map between a simplicial set ir~1cX which classifies certain D-diagrams of simplicial sets associated with X and which was studied in [5], and a simplicial set cX which classifies similar D-diagrams in the homotopy category. As the homotopy types of 7r_1cX and cX can be expressed as homotopy inverse limits of simplicial sets whose homotopy types are quite well understood, the same can therefore be done for rX.
    [Show full text]
  • Classifying Spaces and Brown Representability
    Classifying spaces and Brown representability Andrew Putman Abstract We sketch the proof of the Brown representability theorem and give a few appli- cations of it, the most important being the construction of the classifying space for principal G-bundles. Let G be a topological group and let CW be the homotopy category of based connected CW-complexes. A classifying space for G is a space BG such that for all X ∈ CW, there exists a bijection between the set of based principal G-bundles on X and the set [X, BG] of all homotopy classes of basepoint-preserving maps from X to BG. This determines the homotopy groups of BG in the following way. ∼ Theorem 0.1. For all n ≥ 1, we have πn(BG) = πn−1(G). Remark. For instance, if G is a discrete group, we deduce that BG is a K(G, 1). Proof of Theorem 0.1. A based map Sn → BG is the same as a based principal G-bundle on Sn. Via the clutching construction, this is the same as a based map Sn−1 → G. Theorem 0.1 might suggest to the reader that the following is true. Theorem 0.2. The group G is homotopy equivalent to ΩBG. Proof. Let us give two equivalent descriptions of the map f : G → ΩBG. • A map f : G → ΩBG is the same as a map ΣG → BG, which is the same as a based principal G-bundle on ΣG. There is an obvious choice of such a bundle, again arising from the clutching construction. • More directly, we define f : G → ΩBG as follows.
    [Show full text]
  • Arxiv:2003.07426V2 [Math.CT] 22 Jul 2021 Modifying Some Constructions of Adams [1] for Connected CW Complexes with Base Point
    BROWN REPRESENTABILITY FOR DIRECTED GRAPHS ZACHARY MCGUIRK † AND BYUNGDO PARK ‡ ABSTRACT. We prove that any contravariant functor from the homotopy category of finite directed graphs to abelian groups satisfying the additivity axiom and the Mayer-Vietoris axiom is repre- sentable. 1. INTRODUCTION The homotopy theory of directed graphs is a discrete analogue of homotopy theory in algebraic topology. In topology, a homotopy between two continuous maps is defined by an interpolating family of continuous maps parametrized by a closed interval [0; 1]. Its discrete analogue we study uses a directed line graph keeping track of a discrete change of directed graph maps. See for example Grigor’yan, Lin, Muranov, and Yau [8]. Efforts towards a homotopy theory for graphs extend back to the 1970’s and 1980’s with some results by Gianella [6] and Malle [16]. However, the most recent variant of graph homotopy theory appears to have taken off with a paper by Chen, Yau, and Yeh from 2001 [2], before culminating in the 2014 work of Grigor’yan, Lin, Muranov, and Yau [8]. Recently, there is increased interest in this new notion of homotopy for directed graphs because it was shown by Grigor’yan, Jimenez, Muranov, and Yau in [7] that the path-space homology theory of directed graphs is invariant under this version of graph homotopy. The path-space homology and cohomology theories for directed graphs were studied by Grigor’yan, Lin, Muranov, and Yau [9] and by Grigor’yan, Muranov, and Yau in [10] and [11]. The homology theory developed in the references above is quite natural, computable, and can be non-trivial for degrees greater than one, depending on the lengths of admissible, @-invariant paths in a directed graph.
    [Show full text]