Quasi-category


In mathematics, more specifically category theory, a quasi-category is a generalization of the notion of a category. The study of such generalizations is known as higher category theory.
Quasi-categories were introduced by.
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.
Quasi-categories are certain simplicial sets. Like ordinary categories, they contain objects and morphisms between these objects. 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. 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 is that, as opposed to the standard notion of a category, there should be a mapping space 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.

Definition

By definition, a quasi-category C is a simplicial set satisfying the inner Kan conditions : every inner horn in C, namely a map of simplicial sets where, has a filler, that is, an extension to a map.
The idea is that 2-simplices are supposed to represent commutative triangles. A map 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.
One consequence of the definition is that 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 from sSet to Cat, known as the fundamental category functor, and for a quasi-category C the fundamental category is the same as the homotopy category, i.e..

Examples