Princeton University Press is proud to have published the Annals of Mathematics Studies since 1940. One of the oldest and most respected series in science publishing, it has included many of the most important and influential mathematical works of the twentieth century. The series continues this tradition into the twenty-first century as Princeton looks forward to publishing the major works of the new millennium.

To mark the continued success of the series, all books are again available in paperback. For a complete list of titles, please visit the Princeton University Press web site: press.Princeton. edu. The most recently published volumes include:

This book is a remarkable achievement, and the reviewer thinks it marks the beginning of a major change in algebraic topology.
— Mark Hovey

Mathematical Reviews
- Mark Hovey

This book is a remarkable achievement, and the reviewer thinks it marks the beginning of a major change in algebraic topology.

From the Publisher

"This book is a remarkable achievement, and the reviewer thinks it marks the beginning of a major change in algebraic topology."—Mark Hovey, Mathematical Reviews

This chapter is intended as a general introduction to higher category theory. We begin with what we feel is the most intuitive approach to the subject using topological categories. This approach is easy to understand but difficult to work with when one wishes to perform even simple categorical constructions. As a remedy, we will introduce the more suitable formalism of [infinity]-categories (called weak Kan complexes in [10] and quasi-categories in [43]), which provides a more convenient setting for adaptations of sophisticated category-theoretic ideas. Our goal in 1.1.1 is to introduce both approaches and to explain why they are equivalent to one another. The proof of this equivalence will rely on a crucial result (Theorem 1.1.5.13) which we will prove in 2.2.

Our second objective in this chapter is to give the reader an idea of how to work with the formalism of [infinity]-categories. In 1.2, we will establish a vocabulary which includes [infinity]-categorical analogues (often direct generalizations) of most of the important concepts from ordinary category theory. To keep the exposition brisk, we will postpone the more difficult proofs until later chapters of this book. Our hope is that, after reading this chapter, a reader who does not wish to be burdened with the details will be able to understand (at least in outline) some of the more conceptual ideas described in Chapter 5 and beyond.

1.1 FOUNDATIONS FOR HIGHER CATEGORY THEORY

1.1.1 Goals and Obstacles

Recall that a category C consists of the following data:

(1) A collection {X,Y,Z, ...} whose members are the objects of ITLITL. We typically write X [member of] ITLITL to indicate that X is an object of ITLITL.

(2) For every pair of objects X, Y [member of] ITLITL, a set [Hom.sub.ITLITL](X, Y) of morphisms from X to Y. We will typically write f : X [right arrow] Y to indicate that f [member of] [Hom.sub.ITLITL](X, Y) and say that f is a morphism from X to Y.

(3) For every object X [member of] ITLITL, an identity morphism [id.sub.X] [member of] [Hom.sub.ITLITL](X,X).

(4) For every triple of objects X, Y, Z [member of] ITLITL, a composition map

[Hom.sub.ITLITL](X, Y) x [Hom.sub.ITLITL](Y,Z) [right arrow] [Hom.sub.ITLITL](X,Z).

Given morphisms f : X [right arrow] Y and g : Y [right arrow] Z, we will usually denote the image of the pair (f, g) under the composition map by gf or g [??] f.

These data are furthermore required to satisfy the following conditions, which guarantee that composition is unital and associative:

(5) For every morphism f : X [right arrow] Y, we have [id.sub.Y] [??]f = f = f [??] [id.sub.X] in [Hom.sub.ITLITL](X, Y).

(6) For every triple of composable morphisms

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII],

we have an equality h [??] (g [??] f) = (h [??] g) [??] f in [Hom.sub.ITLITL](W, Z).

The theory of categories has proven to be a valuable organization tool in many areas of mathematics. Mathematical structures of virtually any type can be viewed as the objects of a suitable category ITLITL, where the morphisms in C are given by structure-preserving maps. There is a veritable legion of examples of categories which fit this paradigm:

The category Set whose objects are sets and whose morphisms are maps of sets.

The category Grp whose objects are groups and whose morphisms are group homomorphisms.

The category Top whose objects are topological spaces and whose morphisms are continuous maps.

The category Cat whose objects are (small) categories and whose morphisms are functors. (Recall that a functor F from ITLITL to D is a map which assigns to each object ITLITL [member of] ITLITL another object FC [member of] D, and to each morphism f : ITLITL [right arrow] ITLITL' in ITLITL a morphism F(f) : FC [right arrow] FC' in D, so that F([id.sub.ITLITL]) = [id.sub.FC] and F(g [??] f) = F(g) [??] F(f).)

???

In general, the existence of a morphism f : X [right arrow] Y in a category ITLITL reflects some relationship that exists between the objects X, Y [member of] ITLITL. In some contexts, these relationships themselves become basic objects of study and can be fruitfully organized into categories:

Example 1.1.1.1. Let Grp be the category whose objects are groups and whose morphisms are group homomorphisms. In the theory of groups, one is often concerned only with group homomorphisms up to conjugacy. The relation of conjugacy can be encoded as follows: for every pair of groups G,H [member of] Grp, there is a category Map(G,H) whose objects are group homomorphisms from G to H (that is, elements of [Hom.sub.Grp](G,H)), where a morphism from f : G [right arrow] H to f' : G [right arrow] H is an element h [member of] H such that hf(g)[h.sup.-1] = f'(g) for all g [member of] G. Note that two group homomorphisms f, f' : G [right arrow] H are conjugate if and only if they are isomorphic when viewed as objects of Map(G,H).

Example 1.1.1.2. Let X and Y be topological spaces and let [f.sub.0], [f.sub.1] : X [right arrow] Y be continuous maps. Recall that a homotopy from [f.sub.0] to [f.sub.1] is a continuous map f : X x [0, 1] [right arrow] Y such that f|X x {0} coincides with [f.sub.0] and f|X x {1} coincides with [f.sub.1]. In algebraic topology, one is often concerned not with the category Top of topological spaces but with its homotopy category: that is, the category obtained by identifying those pairs of morphisms [f.sub.0], [f.sub.1] : X [right arrow] Y which are homotopic to one another. For many purposes, it is better to do something a little bit more sophisticated: namely, one can form a category Map(X, Y) whose objects are continuous maps f : X [right arrow] Y and whose morphisms are given by (homotopy classes of) homotopies.

Example 1.1.1.3. Given a pair of categories ITLITL and D, the collection of all functors from ITLITL to D is itself naturally organized into a category Fun(C,D), where the morphisms are given by natural transformations. (Recall that, given a pair of functors F,G : ITLITL [right arrow] D, a natural transformation [alpha] : F [right arrow] G is a collection of morphisms [{[[alpha].sub.ITLITL] : F(ITLITL) [right arrow] G(ITLITL)}.sub.ITLITL [member of] ITLITL] which satisfy the following condition: for every morphism f : ITLITL [right arrow] C' in ITLITL, the diagram

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

commutes in D.)

In each of these examples, the objects of interest can naturally be organized into what is called a 2-category (or bicategory): we have not only a collection of objects and a notion of morphisms between objects but also a notion of morphisms between morphisms, which are called 2-morphisms. The vision of higher category theory is that there should exist a good notion of n-category for all n [greater than or equal to] = 0 in which we have not only objects, morphisms, and 2-morphisms but also k-morphisms for all k [less than or equal to] = n. Finally, in some sort of limit we might hope to obtain a theory of [infinity]-categories, where there are morphisms of all orders.

Example 1.1.1.4. Let X be a topological space and 0 [less than or equal to] n [less than or equal to] [infinity]. We can extract an n-category [pi] [less than or equal to] [sub.n]X (roughly) as follows. The objects of [pi] [less than or equal to] [sub.n]X are the points of X. If x, y [member of] X, then the morphisms from x to y in [pi] [less than or equal to] [sub.n]X are given by continuous paths [0, 1] [right arrow] X starting at x and ending at y. The 2-morphisms are given by homotopies of paths, the 3-morphisms by homotopies between homotopies, and so forth. Finally, if n < [infinity], then two n-morphisms of [pi] [less than or equal to] [sub.n]X are considered to be the same if and only if they are homotopic to one another.

If n = 0, then [pi] [less than or equal to] [sub.n]X can be identified with the set [pi][sub.0]X of path components of X. If n = 1, then our definition of [pi] [less than or equal to] [sub.n]X agrees with the usual definition for the fundamental groupoid of X. For this reason, [pi] [less than or equal to] [sub.n]X is often called the fundamental n-groupoid of X. It is called an n-groupoid (rather than a mere n-category) because every k-morphism of [[pi].sub.[less than or equal to] [sub.k]]X has an inverse (at least up to homotopy).

There are many approaches to realizing the theory of higher categories. We might begin by defining a 2-category to be a "category enriched over ITLITLat." In other words, we consider a collection of objects together with a category of morphisms Hom(A,B) for any two objects A and B and composition functors [[c.sub.ABC] x Hom(A,B) x Hom(B,C) [right arrow] Hom(A,C) (to simplify the discussion, we will ignore identity morphisms for a moment). These functors are required to satisfy an associative law, which asserts that for any quadruple (A,B,C,D) of objects, the diagram

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

commutes; in other words, one has an equality of functors

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

from Hom(A,B) x Hom(B,C) x Hom(C,D) to Hom(A,D). This leads to the definition of a strict 2-category.

At this point, we should object that the definition of a strict 2-category violates one of the basic philosophical principles of category theory: one should never demand that two functors F and F' be equal to one another. Instead one should postulate the existence of a natural isomorphism between F and F'. This means that the associative law should not take the form of an equation but of additional structure: a collection of isomorphisms [[gamma.sub.[ABCD] : [C.sub.ACD] [??]([c.sub.ABC] x 1) [equivalent] [c.sub.ABD] [??](1 x [c.sub.BCD]). We should further demand that the isomorphisms [[gamma.sub.[ABCD] be functorial in the quadruple (A,B,C,D) and satisfy certain higher associativity conditions, which generalize the "Pentagon axiom" described in A.1.3. After formulating the appropriate conditions, we arrive at the definition of a weak 2-category.

Let us contrast the notions of strict 2-category and weak 2-category. The former is easier to define because we do not have to worry about the higher associativity conditions satisfied by the transformations [[gamma].sub.ABCD]. On the other hand, the latter notion seems more natural if we take the philosophy of category theory seriously. In this case, we happen to be lucky: the notions of strict 2-category and weak 2-category turn out to be equivalent. More precisely, any weak 2-category is equivalent (in the relevant sense) to a strict 2-category. The choice of definition can therefore be regarded as a question of aesthetics.

We now plunge onward to 3-categories. Following the above program, we might define a strict 3-category to consist of a collection of objects together with strict 2-categories Hom(A,B) for any pair of objects A and B, together with a strictly associative composition law. Alternatively, we could seek a definition of weak 3-category by allowing Hom(A,B) to be a weak 2-category, requiring associativity only up to natural 2-isomorphisms, which satisfy higher associativity laws up to natural 3-isomorphisms, which in turn satisfy still higher associativity laws of their own. Unfortunately, it turns out that these notions are not equivalent.

Both of these approaches have serious drawbacks. The obvious problem with weak 3-categories is that an explicit definition is extremely complicated (see [33], where a definition is given along these lines), to the point where it is essentially unusable. On the other hand, strict 3-categories have the problem of not being the correct notion: most of the weak 3-categories which occur in nature are not equivalent to strict 3-categories. For example, the fundamental 3-groupoid of the 2-sphere [S.sup.2] cannot be described using the language of strict 3-categories. The situation only gets worse (from either point of view) as we pass to 4-categories and beyond.

Fortunately, it turns out that major simplifications can be introduced if we are willing to restrict our attention to [infinity]-categories in which most of the higher morphisms are invertible. From this point forward, we will use the term ([infinity], n)-category to refer to [infinity]-categories in which all k-morphisms are invertible for k > n. The [infinity]-categories described in Example 1.1.1.4 (when n = [infinity]) are all ([infinity], 0)-categories. The converse, which asserts that every ([infinity], 0)-category has the form [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] for some topological space X, is a generally accepted principle of higher category theory. Moreover, the [infinity]-groupoid [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] encodes the entire homotopy type of X. In other words, ([infinity], 0)-categories (that is, [infinity]-categories in which all morphisms are invertible) have been extensively studied from another point of view: they are essentially the same thing as "spaces" in the sense of homotopy theory, and there are many equivalent ways to describe them (for example, we can use CW complexes or simplicial sets).

Convention 1.1.1.5. We will sometimes refer to ([infinity], 0)-categories as [infinity]-groupoids and ([infinity], 2)-categories as [infinity]-bicategories. Unless we specify otherwise, the generic term "[infinity]-category" will refer to an ([infinity], 1)-category.

In this book, we will restrict our attention almost entirely to the theory of [infinity]-categories (in which we have only invertible n-morphisms for n [greater than or equal to] 2). Our reasons are threefold:

(1) Allowing noninvertible n-morphisms for n > 1 introduces a number of additional complications to the theory at both technical and conceptual levels. As we will see throughout this book, many ideas from category theory generalize to the [infinity]-categorical setting in a natural way. However, these generalizations are not so straightforward if we allow noninvertible 2-morphisms. For example, one must distinguish between strict and lax fiber products, even in the setting of "classical" 2-categories.

(2) For the applications studied in this book, we will not need to consider ([infinity], n)-categories for n > 2. The case n = 2 is of some relevance because the collection of (small) [infinity]-categories can naturally be viewed as a (large) [infinity]-bicategory. However, we will generally be able to exploit this structure in an ad hoc manner without developing any general theory of [infinity]-bicategories.

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## More About This Textbook

## Overview

Princeton University Press is proud to have published the Annals of Mathematics Studies since 1940. One of the oldest and most respected series in science publishing, it has included many of the most important and influential mathematical works of the twentieth century. The series continues this tradition into the twenty-first century as Princeton looks forward to publishing the major works of the new millennium.

To mark the continued success of the series, all books are again available in paperback. For a complete list of titles, please visit the Princeton University Press web site: press.Princeton. edu. The most recently published volumes include:

## Editorial Reviews

## Mathematical Reviews

This book is a remarkable achievement, and the reviewer thinks it marks the beginning of a major change in algebraic topology.— Mark Hovey

## Mathematical Reviews - Mark Hovey

This book is a remarkable achievement, and the reviewer thinks it marks the beginning of a major change in algebraic topology.## From the Publisher

"This book is a remarkable achievement, and the reviewer thinks it marks the beginning of a major change in algebraic topology."—Mark Hovey,

Mathematical Reviews## Product Details

## Related Subjects

## Meet the Author

Jacob Lurie is associate professor of mathematics at Massachusetts Institute of Technology.

## Read an Excerpt

## Higher Topos Theory

By Jacob LuriePRINCETON UNIVERSITY PRESSCopyright © 2009Princeton University PressAll right reserved.

ISBN: 978-0-691-14049-0## Chapter One

An Overview of Higher Category TheoryThis chapter is intended as a general introduction to higher category theory. We begin with what we feel is the most intuitive approach to the subject using

topological categories. This approach is easy to understand but difficult to work with when one wishes to perform even simple categorical constructions. As a remedy, we will introduce the more suitable formalism of [infinity]-categories(calledweak Kan complexesin [10] andquasi-categoriesin [43]), which provides a more convenient setting for adaptations of sophisticated category-theoretic ideas. Our goal in 1.1.1 is to introduce both approaches and to explain why they are equivalent to one another. The proof of this equivalence will rely on a crucial result (Theorem 1.1.5.13) which we will prove in 2.2.Our second objective in this chapter is to give the reader an idea of how to work with the formalism of [infinity]-categories. In 1.2, we will establish a vocabulary which includes [infinity]-categorical analogues (often direct generalizations) of most of the important concepts from ordinary category theory. To keep the exposition brisk, we will postpone the more difficult proofs until later chapters of this book. Our hope is that, after reading this chapter, a reader who does not wish to be burdened with the details will be able to understand (at least in outline) some of the more conceptual ideas described in Chapter 5 and beyond.

1.1 FOUNDATIONS FOR HIGHER CATEGORY THEORY1.1.1 Goals and ObstaclesRecall that a

category Cconsists of the following data:(1) A collection {

X,Y,Z, ...} whose members are theobjectsof ITLITL. We typically write X [member of] ITLITL to indicate thatXis an object of ITLITL.(2) For every pair of objects

X, Y[member of] ITLITL, a set [Hom.sub.ITLITL](X, Y) ofmorphismsfromXtoY. We will typically writef : X[right arrow]Yto indicate thatf[member of] [Hom.sub.ITLITL](X, Y) and say thatf is a morphism from X to Y.(3) For every object

X[member of] ITLITL, anidentity morphism[id.sub.X] [member of] [Hom.sub.ITLITL](X,X).(4) For every triple of objects

X, Y, Z[member of] ITLITL, a composition map[Hom.sub.ITLITL](

X, Y) x [Hom.sub.ITLITL](Y,Z) [right arrow] [Hom.sub.ITLITL](X,Z).Given morphisms

f:X[right arrow]Yandg:Y[right arrow]Z, we will usually denote the image of the pair (f, g) under the composition map bygforg[??]f.These data are furthermore required to satisfy the following conditions, which guarantee that composition is unital and associative:

(5) For every morphism

f:X[right arrow]Y, we have [id.sub.Y] [??]f = f = f[??] [id.sub.X] in [Hom.sub.ITLITL](X, Y).(6) For every triple of composable morphisms

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII],

we have an equality

h[??] (g[??]f) = (h[??]g) [??]fin [Hom.sub.ITLITL](W, Z).The theory of categories has proven to be a valuable organization tool in many areas of mathematics. Mathematical structures of virtually any type can be viewed as the objects of a suitable category ITLITL, where the morphisms in C are given by structure-preserving maps. There is a veritable legion of examples of categories which fit this paradigm:

The category Set whose objects are sets and whose morphisms are maps of sets.

The category

Grp whose objects are groups and whose morphisms are group homomorphisms.The category Top whose objects are topological spaces and whose morphisms are continuous maps.

The category Cat whose objects are (small) categories and whose morphisms are functors. (Recall that a functor

Ffrom ITLITL toDis a map which assigns to each object ITLITL [member of] ITLITL another objectFC[member of]D, and to each morphismf: ITLITL [right arrow] ITLITL' in ITLITL a morphismF(f) :FC[right arrow]FC'inD, so thatF([id.sub.ITLITL]) = [id.sub.FC] andF(g [??]f) =F(g) [??]F(f).)???

In general, the existence of a morphism

f:X[right arrow]Yin a category ITLITL reflects some relationship that exists between the objectsX, Y[member of] ITLITL. In some contexts, these relationships themselves become basic objects of study and can be fruitfully organized into categories:Example 1.1.1.1.LetGrp be the category whose objects are groups and whose morphisms are group homomorphisms. In the theory of groups, one is often concerned only with group homomorphismsup to conjugacy. The relation of conjugacy can be encoded as follows: for every pair of groupsG,H[member of]Grp, there is a category Map(G,H) whose objects are group homomorphisms fromGtoH(that is, elements of [Hom.sub.Grp](G,H)), where a morphism fromf:G[right arrow]Htof' :G[right arrow]His an elementh[member of]Hsuch thathf(g)[h.sup.-1] =f'(g) for allg[member of]G. Note that two group homomorphismsf,f':G[right arrow]Hare conjugate if and only if they are isomorphic when viewed as objects of Map(G,H).Example 1.1.1.2.LetXandYbe topological spaces and let [f.sub.0], [f.sub.1] :X[right arrow]Ybe continuous maps. Recall that ahomotopyfrom [f.sub.0] to [f.sub.1] is a continuous mapf:Xx [0, 1] [right arrow]Ysuch thatf|Xx {0} coincides with [f.sub.0] andf|Xx {1} coincides with [f.sub.1]. In algebraic topology, one is often concerned not with the category Top of topological spaces but with itshomotopy category: that is, the category obtained by identifying those pairs of morphisms [f.sub.0], [f.sub.1] :X[right arrow]Ywhich are homotopic to one another. For many purposes, it is better to do something a little bit more sophisticated: namely, one can form a category Map(X, Y) whose objects are continuous mapsf:X[right arrow]Yand whose morphisms are given by (homotopy classes of) homotopies.Example 1.1.1.3.Given a pair of categories ITLITL andD, the collection of all functors from ITLITL toDis itself naturally organized into a category Fun(C,D), where the morphisms are given bynatural transformations. (Recall that, given a pair of functorsF,G: ITLITL [right arrow]D, a natural transformation [alpha] :F[right arrow]Gis a collection of morphisms [{[[alpha].sub.ITLITL] :F(ITLITL) [right arrow]G(ITLITL)}.sub.ITLITL [member of] ITLITL] which satisfy the following condition: for every morphismf: ITLITL [right arrow]C'in ITLITL, the diagram[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

commutes in

D.)In each of these examples, the objects of interest can naturally be organized into what is called a 2-

category(orbicategory): we have not only a collection of objects and a notion of morphisms between objects but also a notion of morphisms between morphisms, which are called 2-morphisms. The vision of higher category theory is that there should exist a good notion ofn-category for alln[greater than or equal to] = 0 in which we have not only objects, morphisms, and 2-morphisms but alsok-morphisms for allk[less than or equal to] =n. Finally, in some sort of limit we might hope to obtain a theory of [infinity]-categories, where there are morphisms of all orders.Example 1.1.1.4.LetXbe a topological space and 0 [less than or equal to]n[less than or equal to] [infinity]. We can extract ann-category [pi] [less than or equal to] [sub.n]X(roughly) as follows. The objects of [pi] [less than or equal to] [sub.n]Xare the points ofX. Ifx, y[member of]X, then the morphisms fromxtoyin [pi] [less than or equal to] [sub.n]Xare given by continuous paths [0, 1] [right arrow]Xstarting atxand ending aty. The 2-morphisms are given by homotopies of paths, the 3-morphisms by homotopies between homotopies, and so forth. Finally, ifn< [infinity], then twon-morphisms of [pi] [less than or equal to] [sub.n]Xare considered to be the same if and only if they are homotopic to one another.If

n= 0, then [pi] [less than or equal to] [sub.n]Xcan be identified with the set [pi][sub.0]Xof path components ofX. Ifn= 1, then our definition of [pi] [less than or equal to] [sub.n]Xagrees with the usual definition for the fundamental groupoid ofX. For this reason, [pi] [less than or equal to] [sub.n]Xis often called thefundamental n-groupoid of X. It is called ann-groupoid(rather than a meren-category) because everyk-morphism of [[pi].sub.[less than or equal to] [sub.k]]Xhas an inverse (at least up to homotopy).There are many approaches to realizing the theory of higher categories. We might begin by defining a 2-category to be a "category enriched over ITLITLat." In other words, we consider a collection of objects together with a

categoryof morphisms Hom(A,B) for any two objectsAandBand compositionfunctors [[c.sub.ABC] x Hom(A,B) x Hom(B,C) [right arrow] Hom(A,C) (to simplify the discussion, we will ignore identity morphisms for a moment). These functors are required to satisfy an associative law, which asserts that for any quadruple (A,B,C,D) of objects, the diagram[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

commutes; in other words, one has an

equalityof functors[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

from Hom(

A,B) x Hom(B,C) x Hom(C,D) to Hom(A,D). This leads to the definition of astrict2-category.At this point, we should object that the definition of a strict 2-category violates one of the basic philosophical principles of category theory: one should never demand that two functors

FandF'be equal to one another. Instead one should postulate the existence of a natural isomorphism betweenFandF'. This means that the associative law should not take the form of an equation but of additional structure: a collection of isomorphisms [[gamma.sub.[ABCD] : [C.sub.ACD] [??]([c.sub.ABC] x 1) [equivalent] [c.sub.ABD] [??](1 x [c.sub.BCD]). We should further demand that the isomorphisms [[gamma.sub.[ABCD] be functorial in the quadruple (A,B,C,D) and satisfy certain higher associativity conditions, which generalize the "Pentagon axiom" described in A.1.3. After formulating the appropriate conditions, we arrive at the definition of aweak2-category.Let us contrast the notions of strict 2-category and weak 2-category. The former is easier to define because we do not have to worry about the higher associativity conditions satisfied by the transformations [[gamma].sub.

ABCD]. On the other hand, the latter notion seems more natural if we take the philosophy of category theory seriously. In this case, we happen to be lucky: the notions of strict 2-category and weak 2-category turn out to be equivalent. More precisely, any weak 2-category is equivalent (in the relevant sense) to a strict 2-category. The choice of definition can therefore be regarded as a question of aesthetics.We now plunge onward to 3-categories. Following the above program, we might define a

strict3-categoryto consist of a collection of objects together with strict 2-categories Hom(A,B) for any pair of objectsAandB, together with a strictly associative composition law. Alternatively, we could seek a definition ofweak3-categoryby allowing Hom(A,B) to be a weak 2-category, requiring associativity only up to natural 2-isomorphisms, which satisfy higher associativity laws up to natural 3-isomorphisms, which in turn satisfy still higher associativity laws of their own. Unfortunately, it turns out that these notions arenotequivalent.Both of these approaches have serious drawbacks. The obvious problem with weak 3-categories is that an explicit definition is extremely complicated (see [33], where a definition is given along these lines), to the point where it is essentially unusable. On the other hand, strict 3-categories have the problem of not being the correct notion: most of the weak 3-categories which occur in nature are not equivalent to strict 3-categories. For example, the fundamental 3-groupoid of the 2-sphere [

S.sup.2] cannot be described using the language of strict 3-categories. The situation only gets worse (from either point of view) as we pass to 4-categories and beyond.Fortunately, it turns out that major simplifications can be introduced if we are willing to restrict our attention to [infinity]-categories in which most of the higher morphisms are invertible. From this point forward, we will use the term ([infinity],

n)-categoryto refer to [infinity]-categories in which allk-morphisms are invertible fork>n. The [infinity]-categories described in Example 1.1.1.4 (whenn= [infinity]) are all ([infinity], 0)-categories. The converse, which asserts that every ([infinity], 0)-category has the form [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] for some topological spaceX, is a generally accepted principle of higher category theory. Moreover, the [infinity]-groupoid [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] encodes the entire homotopy type ofX. In other words, ([infinity], 0)-categories (that is, [infinity]-categories in whichallmorphisms are invertible) have been extensively studied from another point of view: they are essentially the same thing as "spaces" in the sense of homotopy theory, and there are many equivalent ways to describe them (for example, we can use CW complexes or simplicial sets).Convention 1.1.1.5.We will sometimes refer to ([infinity], 0)-categories as [infinity]-groupoidsand ([infinity], 2)-categories as [infinity]-bicategories. Unless we specify otherwise, the generic term "[infinity]-category" will refer to an ([infinity], 1)-category.In this book, we will restrict our attention almost entirely to the theory of [infinity]-categories (in which we have only invertible

n-morphisms forn[greater than or equal to] 2). Our reasons are threefold:(1) Allowing noninvertible

n-morphisms forn> 1 introduces a number of additional complications to the theory at both technical and conceptual levels. As we will see throughout this book, many ideas from category theory generalize to the [infinity]-categorical setting in a natural way. However, these generalizations are not so straightforward if we allow noninvertible 2-morphisms. For example, one must distinguish between strict and lax fiber products, even in the setting of "classical" 2-categories.(2) For the applications studied in this book, we will not need to consider ([infinity],

n)-categories forn> 2. The casen= 2 is of some relevance because the collection of (small) [infinity]-categories can naturally be viewed as a (large) [infinity]-bicategory. However, we will generally be able to exploit this structure in an ad hoc manner without developing any general theory of [infinity]-bicategories.(Continues...)## Table of Contents

Preface vii

Chapter 1 An Overview of Higher Category Theory 1

1.1 Foundations for Higher Category Theory 1

1.2 The Language of Higher Category Theory 26

Chapter 2 Fibrations of Simplicial Sets 53

2.1 Left Fibrations 55

2.2 Simplicial Categories and &infinity;-Categories 72

2.3 Inner Fibrations 95

2.4 Cartesian Fibrations 114

Chapter 3 The &infinity;-Category of &infinity;-Categories 145

3.1 Marked Simplicial Sets 147

3.2 Straightening and Unstraightening 169

3.3 Applications 204

Chapter 4 Limits and Colimits 223

4.1 Cofinality 223

4.2 Techniques for Computing Colimits 240

4.3 Kan Extensions 261

4.4 Examples of Colimits 292

Chapter 5 Presentable and Accessible &infinity;-Categories 311

5.1 &infinity;-Categories of Presheaves 312

5.2 Adjoint Functors 331

5.3 &infinity;-Categories of Inductive Limits 377

5.4 Accessible &infinity;-Categories 414

5.5 Presentable &infinity;-Categories 455

Chapter 6 &infinity;-Topoi 526

6.1 &infinity;-Topoi: Definitions and Characterizations 527

6.2 Constructions of &infinity;-Topoi 569

6.3 The &infinity;-Category of &infinity;-Topoi 593

6.4 &infinity;-Topoi 632

6.5 Homotopy Theory in an &infinity;-Topos 651

Chapter 7 Higher Topos Theory in Topology 682

7.1 Paracompact Spaces 683

7.2 Dimension Theory 711

7.3 The Proper Base Change Theorem 742

Appendix 781

A.1 Category Theory 781

A.2 Model Categories 803

A.3 Simplicial Categories 844

Bibliography 909

General Index 915

Index of Notation 923