Fields Lectures: Simplicial Presheaves
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Sheaves and Homotopy Theory
SHEAVES AND HOMOTOPY THEORY DANIEL DUGGER The purpose of this note is to describe the homotopy-theoretic version of sheaf theory developed in the work of Thomason [14] and Jardine [7, 8, 9]; a few enhancements are provided here and there, but the bulk of the material should be credited to them. Their work is the foundation from which Morel and Voevodsky build their homotopy theory for schemes [12], and it is our hope that this exposition will be useful to those striving to understand that material. Our motivating examples will center on these applications to algebraic geometry. Some history: The machinery in question was invented by Thomason as the main tool in his proof of the Lichtenbaum-Quillen conjecture for Bott-periodic algebraic K-theory. He termed his constructions `hypercohomology spectra', and a detailed examination of their basic properties can be found in the first section of [14]. Jardine later showed how these ideas can be elegantly rephrased in terms of model categories (cf. [8], [9]). In this setting the hypercohomology construction is just a certain fibrant replacement functor. His papers convincingly demonstrate how many questions concerning algebraic K-theory or ´etale homotopy theory can be most naturally understood using the model category language. In this paper we set ourselves the specific task of developing some kind of homotopy theory for schemes. The hope is to demonstrate how Thomason's and Jardine's machinery can be built, step-by-step, so that it is precisely what is needed to solve the problems we encounter. The papers mentioned above all assume a familiarity with Grothendieck topologies and sheaf theory, and proceed to develop the homotopy-theoretic situation as a generalization of the classical case. -
Simplicial Sets, Nerves of Categories, Kan Complexes, Etc
SIMPLICIAL SETS, NERVES OF CATEGORIES, KAN COMPLEXES, ETC FOLING ZOU These notes are taken from Peter May's classes in REU 2018. Some notations may be changed to the note taker's preference and some detailed definitions may be skipped and can be found in other good notes such as [2] or [3]. The note taker is responsible for any mistakes. 1. simplicial approach to defining homology Defnition 1. A simplical set/group/object K is a sequence of sets/groups/objects Kn for each n ≥ 0 with face maps: di : Kn ! Kn−1; 0 ≤ i ≤ n and degeneracy maps: si : Kn ! Kn+1; 0 ≤ i ≤ n satisfying certain commutation equalities. Images of degeneracy maps are said to be degenerate. We can define a functor: ordered abstract simplicial complex ! sSet; K 7! Ks; where s Kn = fv0 ≤ · · · ≤ vnjfv0; ··· ; vng (may have repetition) is a simplex in Kg: s s Face maps: di : Kn ! Kn−1; 0 ≤ i ≤ n is by deleting vi; s s Degeneracy maps: si : Kn ! Kn+1; 0 ≤ i ≤ n is by repeating vi: In this way it is very straightforward to remember the equalities that face maps and degeneracy maps have to satisfy. The simplical viewpoint is helpful in establishing invariants and comparing different categories. For example, we are going to define the integral homology of a simplicial set, which will agree with the simplicial homology on a simplical complex, but have the virtue of avoiding the barycentric subdivision in showing functoriality and homotopy invariance of homology. This is an observation made by Samuel Eilenberg. To start, we construct functors: F C sSet sAb ChZ: The functor F is the free abelian group functor applied levelwise to a simplical set. -
The Simplicial Parallel of Sheaf Theory Cahiers De Topologie Et Géométrie Différentielle Catégoriques, Tome 10, No 4 (1968), P
CAHIERS DE TOPOLOGIE ET GÉOMÉTRIE DIFFÉRENTIELLE CATÉGORIQUES YUH-CHING CHEN Costacks - The simplicial parallel of sheaf theory Cahiers de topologie et géométrie différentielle catégoriques, tome 10, no 4 (1968), p. 449-473 <http://www.numdam.org/item?id=CTGDC_1968__10_4_449_0> © Andrée C. Ehresmann et les auteurs, 1968, tous droits réservés. L’accès aux archives de la revue « Cahiers de topologie et géométrie différentielle catégoriques » implique l’accord avec les conditions générales d’utilisation (http://www.numdam.org/conditions). Toute utilisation commerciale ou impression systématique est constitutive d’une infraction pénale. Toute copie ou impression de ce fichier doit contenir la présente mention de copyright. Article numérisé dans le cadre du programme Numérisation de documents anciens mathématiques http://www.numdam.org/ CAHIERS DE TOPOLOGIE ET GEOMETRIE DIFFERENTIELLE COSTACKS - THE SIMPLICIAL PARALLEL OF SHEAF THEORY by YUH-CHING CHEN I NTRODUCTION Parallel to sheaf theory, costack theory is concerned with the study of the homology theory of simplicial sets with general coefficient systems. A coefficient system on a simplicial set K with values in an abe - . lian category 8 is a functor from K to Q (K is a category of simplexes) ; it is called a precostack; it is a simplicial parallel of the notion of a presheaf on a topological space X . A costack on K is a « normalized» precostack, as a set over a it is realized simplicial K/" it is simplicial « espace 8ta18 ». The theory developed here is functorial; it implies that all >> homo- logy theories are derived functors. Although the treatment is completely in- dependent of Topology, it is however almost completely parallel to the usual sheaf theory, and many of the same theorems will be found in it though the proofs are usually quite different. -
The 2-Category Theory of Quasi-Categories
THE 2-CATEGORY THEORY OF QUASI-CATEGORIES EMILY RIEHL AND DOMINIC VERITY Abstract. In this paper we re-develop the foundations of the category theory of quasi- categories (also called 1-categories) using 2-category theory. We show that Joyal’s strict 2-category of quasi-categories admits certain weak 2-limits, among them weak comma objects. We use these comma quasi-categories to encode universal properties relevant to limits, colimits, and adjunctions and prove the expected theorems relating these notions. These universal properties have an alternate form as absolute lifting diagrams in the 2- category, which we show are determined pointwise by the existence of certain initial or terminal vertices, allowing for the easy production of examples. All the quasi-categorical notions introduced here are equivalent to the established ones but our proofs are independent and more “formal”. In particular, these results generalise immediately to model categories enriched over quasi-categories. Contents 1. Introduction 2 1.1. A generalisation 3 1.2. Outline 4 1.3. Acknowledgments 5 2. Background on quasi-categories 6 2.1. Some standard simplicial notation 6 2.2. Quasi-categories 8 2.3. Isomorphisms and marked simplicial sets 10 2.4. Join and slice 14 3. The 2-category of quasi-categories 19 3.1. Relating 2-categories and simplicially enriched categories 19 3.2. The 2-category of quasi-categories 21 3.3. Weak 2-limits 24 arXiv:1306.5144v4 [math.CT] 6 May 2015 3.4. Slices of the category of quasi-categories 33 3.5. A strongly universal characterisation of weak comma objects 37 4. -
Quasicategories 1.1 Simplicial Sets
Quasicategories 12 November 2018 1.1 Simplicial sets We denote by ∆ the category whose objects are the sets [n] = f0; 1; : : : ; ng for n ≥ 0 and whose morphisms are order-preserving functions [n] ! [m]. A simplicial set is a functor X : ∆op ! Set, where Set denotes the category of sets. A simplicial map f : X ! Y between simplicial sets is a natural transforma- op tion. The category of simplicial sets with simplicial maps is denoted by Set∆ or, more concisely, as sSet. For a simplicial set X, we normally write Xn instead of X[n], and call it the n set of n-simplices of X. There are injections δi :[n − 1] ! [n] forgetting i and n surjections σi :[n + 1] ! [n] repeating i for 0 ≤ i ≤ n that give rise to functions n n di : Xn −! Xn−1; si : Xn+1 −! Xn; called faces and degeneracies respectively. Since every order-preserving function [n] ! [m] is a composite of a surjection followed by an injection, the sets fXngn≥0 k ` together with the faces di and degeneracies sj determine uniquely a simplicial set X. Faces and degeneracies satisfy the simplicial identities: n−1 n n−1 n di ◦ dj = dj−1 ◦ di if i < j; 8 sn−1 ◦ dn if i < j; > j−1 i n+1 n < di ◦ sj = idXn if i = j or i = j + 1; :> n−1 n sj ◦ di−1 if i > j + 1; n+1 n n+1 n si ◦ sj = sj+1 ◦ si if i ≤ j: For n ≥ 0, the standard n-simplex is the simplicial set ∆[n] = ∆(−; [n]), that is, ∆[n]m = ∆([m]; [n]) for all m ≥ 0. -
A Model Structure for Quasi-Categories
A MODEL STRUCTURE FOR QUASI-CATEGORIES EMILY RIEHL DISCUSSED WITH J. P. MAY 1. Introduction Quasi-categories live at the intersection of homotopy theory with category theory. In particular, they serve as a model for (1; 1)-categories, that is, weak higher categories with n-cells for each natural number n that are invertible when n > 1. Alternatively, an (1; 1)-category is a category enriched in 1-groupoids, e.g., a topological space with points as 0-cells, paths as 1-cells, homotopies of paths as 2-cells, and homotopies of homotopies as 3-cells, and so forth. The basic data for a quasi-category is a simplicial set. A precise definition is given below. For now, a simplicial set X is given by a diagram in Set o o / X o X / X o ··· 0 o / 1 o / 2 o / o o / with certain relations on the arrows. Elements of Xn are called n-simplices, and the arrows di : Xn ! Xn−1 and si : Xn ! Xn+1 are called face and degeneracy maps, respectively. Intuition is provided by simplical complexes from topology. There is a functor τ1 from the category of simplicial sets to Cat that takes a simplicial set X to its fundamental category τ1X. The objects of τ1X are the elements of X0. Morphisms are generated by elements of X1 with the face maps defining the source and target and s0 : X0 ! X1 picking out the identities. Composition is freely generated by elements of X1 subject to relations given by elements of X2. More specifically, if x 2 X2, then we impose the relation that d1x = d0x ◦ d2x. -
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: -
Exposé I – Elements of Parametrized Higher Category Theory
PARAMETRIZED HIGHER CATEGORY THEORY AND HIGHER ALGEBRA: EXPOSÉ I – ELEMENTS OF PARAMETRIZED HIGHER CATEGORY THEORY CLARK BARWICK, EMANUELE DOTTO, SAUL GLASMAN, DENIS NARDIN, AND JAY SHAH Abstract. We introduce the basic elements of the theory of parametrized ∞-categories and functors between them. These notions are defined as suitable fibrations of ∞-categories and functors between them. We give as many examples as we are able at this stage. Simple operations, such as the formation of opposites and the formation of functor ∞-categories, become slightly more involved in the parametrized setting, but we explain precisely how to perform these constructions. All of these constructions can be performed explicitly, without resorting to such acts of desperation as straightening. The key results of this Exposé are: (1) a universal characterization of the 푇-∞-category of 푇-objects in any ∞-category, (2) the existence of an internal Hom for 푇-∞-categories, and (3) a parametrized Yoneda lemma. Contents 1. Parametrized ∞-categories 1 2. Examples of parametrized ∞-categories 3 3. Parametrized opposites 7 4. Parametrized subcategories 7 5. Constructing 푇-∞-categories via pairings 8 6. A technical result: the strong pushforward 9 7. 푇-objects in ∞-categories 11 8. Parametrized fibrations 14 9. Parametrized functor categories 15 10. The parametrized Yoneda embedding 19 Appendix A. Notational glossary 20 References 21 1. Parametrized ∞-categories Suppose 퐺 a finite group. At a minimum, a 퐺-∞-category should consist of an ∞- category 퐶 along with a weak action 휌 of 퐺. In particular, for every element 푔 ∈ 퐺, one should have an equivalence 휌(푔)∶ 퐶 ∼ 퐶, and for every 푔, ℎ ∈ 퐺, one should have a natu- ral equivalence 휌(푔ℎ) ≃ 휌(푔) ∘ 휌(ℎ), and these natural equivalences should then in turn be constrained by an infinite family of homotopies that express the higher associativity of 휌. -
A Simplicial Set Is a Functor X : a Op → Set, Ie. a Contravariant Set-Valued
Contents 9 Simplicial sets 1 10 The simplex category and realization 10 11 Model structure for simplicial sets 15 9 Simplicial sets A simplicial set is a functor X : Dop ! Set; ie. a contravariant set-valued functor defined on the ordinal number category D. One usually writes n 7! Xn. Xn is the set of n-simplices of X. A simplicial map f : X ! Y is a natural transfor- mation of such functors. The simplicial sets and simplicial maps form the category of simplicial sets, denoted by sSet — one also sees the notation S for this category. If A is some category, then a simplicial object in A is a functor A : Dop ! A : Maps between simplicial objects are natural trans- formations. 1 The simplicial objects in A and their morphisms form a category sA . Examples: 1) sGr = simplicial groups. 2) sAb = simplicial abelian groups. 3) s(R − Mod) = simplicial R-modules. 4) s(sSet) = s2Set is the category of bisimplicial sets. Simplicial objects are everywhere. Examples of simplicial sets: 1) We’ve already met the singular set S(X) for a topological space X, in Section 4. S(X) is defined by the cosimplicial space (covari- ant functor) n 7! jDnj, by n S(X)n = hom(jD j;X): q : m ! n defines a function ∗ n q m S(X)n = hom(jD j;X) −! hom(jD j;X) = S(X)m by precomposition with the map q : jDmj ! jDmj. The assignment X 7! S(X) defines a covariant func- tor S : CGWH ! sSet; called the singular functor. -
Local Complete Segal Spaces Arxiv:1607.05794V4 [Math.CT] 2 Sep 2018
Local Complete Segal Spaces Nicholas Meadows November 7, 2018 1 Introduction The purpose of this paper is to develop a model structure on bi-simplicial presheaves in which the weak equivalences are stalkwise equivalences in the complete Segal model structure on bi-simplicial sets, and show that it is Quillen equivalent to the local Joyal model structure on simplicial presheaves of [9]. The existence of the local complete Segal model structure was con- jectured in [10, Section 1.3]. The technique of Boolean localization is used extensively to develop this model structure (c.f. [4] and [9]). This is the second in a series of three papers, including [9] and [8], which establish local analogues of three of the main extant models of higher category theory and establish a series of Quillen equivalences connecting them. The long-term objectives of this project are to apply these results to Simpson's theory of higher stacks, as discussed in [11], and study variants of non-abelian cohomology (c.f. [8, Section 5] for an initial thrust in this direction). As such, we have modelled our approach to local higher category theory on [4]; this book contains numerous applications of local homotopy theory to geometric phenomena. In Section 2, we establish notational and terminology conventions. In arXiv:1607.05794v4 [math.CT] 2 Sep 2018 Section 3 of the paper, we review some properties of the complete Segal model structure, as well as describe a variety of Quillen adjunctions between the complete Segal model structure, Joyal model structure, and standard model structure. These results are necessary for establishing the main results of the paper. -
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. -
Simplicial Presheaves
journal of Pure and Applied Algebra 47 (1987) 35-87 35 N0rth-Holland SIMPLICIAL PRESHEAVES J.F. JARDINE* Mathematics Department, University of Western Ontario, London, Ontario N6A 5B7, Canada Communicated by E.M. Friedlander Received 5 October 1985 Revised January 1986 istroduction The central organizational theorem of simplicial homotopy theory asserts that the category S of simplicial sets has a closed model structure. This means that S comes equipped with three classes of morphisms, namely cofibrations (inclusions), fibra- tions (Kan fibrations) and weak equivalences (maps which induce l~omotopy equivalences of realizations), which together satisfy Quillen's closed model axioms CM1 to CMS. This theorem is well known and widely used (see [23] and [3]). One could reasonably ask for such a theorem for the category SShv(C) of simpli- dal sheaves on a Grothendieck site C, based on the intuition that any theorem which is true for sets should be true for topoi. Immediately, however, a dilemma presents itself. On the one hand, cohomological considerations, like the Verdier hyper- covering theorem, suggest a local theory of Kan fibrations. For example, if opl r is the site of open subsets of a topological space T, then a map f: X-, Y of simplicial sheaves should be a local fibration if and only if each map of stalks fx: Xx--" Yx is a Kan fibration in the usual sense. On the other hand, monomorphisms surely should be cofibrations, giving a global theory. The two approaches do, in fact, yield axiomatic homotopy theories for all categories of simplicial sheaves. The local theory for the category of simplicial sheaves on a topological space was constructed by Brown [4]; the corresponding global theory was developed slightly later by Brown and Gersten [5].