In mathematics, more specifically category theory, a quasi-category (also called quasicategory, weak Kan complex, inner Kan complex, infinity category, ∞-category, Boardman complex, quategory) is a generalization of the notion of a category. The study of such generalizations is known as higher category theory.
Overview Quasi-categories were introduced by Boardman & Vogt (1973). André Joyal has much advanced the study of quasi-categories showing that most of the usual basic category theory and some of the advanced notions and theorems have their analogues for quasi-categories. An elaborate treatise of the theory of quasi-categories has been expounded by Jacob Lurie (2009). Quasi-categories are certain simplicial sets. Like ordinary categories, they contain objects (the 0-simplices of the simplicial set) and morphisms between these objects (1-simplices). But unlike categories, the composition of two morphisms need not be uniquely defined. All the morphisms that can serve as composition of two given morphisms are related to each other by higher order invertible morphisms (2-simplices thought of as "homotopies"). These higher order morphisms can also be composed, but again the composition is well-defined only up to still higher order invertible morphisms, etc. The idea of higher category theory (at least, higher category theory when higher morphisms are invertible) is that, as opposed to the standard notion of a category, there should be a mapping space (rather than a mapping set) between two objects. This suggests that a higher category should simply be a topologically enriched category. The model of quasi-categories is, however, better suited to applications than that of topologically enriched categories, though it has been proved by Lurie that the two have natural model structures that are Quillen equivalent (see § Homotopy coherent nerve).
Definition By definition, a quasi-category C is a simplicial set satisfying the inner Kan conditions (also called weak Kan condition): every inner horn in C, namely a map of simplicial sets Λ k [ n ] → C {\displaystyle \Lambda ^{k}[n]\to C} where 0 < k < n {\displaystyle 0<k<n} , has a filler, that is, an extension to a map Δ [ n ] → C {\displaystyle \Delta [n]\to C} . (See Kan fibration#Definitions for a definition of the simplicial sets Δ [ n ] {\displaystyle \Delta [n]} and Λ k [ n ] {\displaystyle \Lambda ^{k}[n]} .) The idea is that 2-simplices Δ [ 2 ] → C {\displaystyle \Delta [2]\to C} are supposed to represent commutative triangles (at least up to homotopy). A map Λ 1 [ 2 ] → C {\displaystyle \Lambda ^{1}[2]\to C} represents a composable pair. Thus, in a quasi-category, one cannot define a composition law on morphisms, since one can choose many ways to compose maps. According to Joyal, one consequence of the definition is that C Δ [ 2 ] → C Λ 1 [ 2 ] {\displaystyle C^{\Delta [2]}\to C^{\Lambda ^{1}[2]}} is a trivial Kan fibration. In other words, while the composition law is not uniquely defined, it is unique up to a contractible choice.
The homotopy category
Given a quasi-category C, one can associate to it an ordinary category hC, called the homotopy category of C. The homotopy category has as objects the vertices of C. The morphisms are given by homotopy classes of edges between vertices. Composition is given using the horn filler condition for n = 2. For a general simplicial set there is a functor τ {\displaystyle \tau } from sSet to Cat, the left-adjoint of the nerve functor, and for a quasi-category C, we have τ ( C ) = h C {\displaystyle \tau (C)=hC} .
Examples The nerve of a category is a quasi-category with the extra property that the filling of any inner horn is unique. Conversely a quasi-category such that any inner horn has a unique filling is isomorphic to the nerve of some category. The homotopy category of the nerve of C is isomorphic to C. Given a topological space X, one can define its singular set S(X), also known as the fundamental ∞-groupoid of X. S(X) is a quasi-category in which every morphism is invertible. The homotopy category of S(X) is the fundamental groupoid of X. More general than the previous example, every Kan complex is an example of a quasi-category. In a Kan complex all maps from all horns—not just inner ones—can be filled, which again has the consequence that all morphisms in a Kan complex are invertible. Kan complexes are thus analogues to groupoids - the nerve of a category is a Kan complex iff the category is a groupoid. Kan complexes themselves form an ∞-category denoted as Kan or also S. Precisely, it is the homotopy coherent nerve of the category of Kan complexes (see also § Homotopy coherent nerve). Similarly, the ∞-category of (small) ∞-categories is defined as the homotopy coherent nerve of the category of ∞-categories. Precisely, let K be the simplicially-enriched category where an object is a small ∞-category and the hom-simplicial-set from C to D is the core of the ∞-category Hom _ ( C , D ) {\displaystyle {\underline {\operatorname {Hom} }}(C,D)} . Then the homotopy coherent nerve of K is the ∞-category of small ∞-categories.
Homotopy coherent nerve
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