1-category

- homotopy hypothesis-theorem
- delooping hypothesis-theorem
- periodic table
- stabilization hypothesis-theorem
- exactness hypothesis
- holographic principle

- (n,r)-category
- Theta-space
- ∞-category/ω-category
- (∞,n)-category
- (∞,2)-category
- (∞,1)-category
- (∞,0)-category/∞-groupoid
- n-category = (n,n)-category
- n-poset = (n-1,n)-category
- n-groupoid = (n,0)-category

- categorification/decategorification
- geometric definition of higher category
- algebraic definition of higher category
- stable homotopy theory

In the context of higher category theory one sometimes needs, for emphasis, to say *1-category* for *category*.

Fix a meaning of $\infty$-category, however weak or strict you wish. Then a **$1$-category** is an $\infty$-category such that every 2-morphism is an equivalence and all parallel pairs of j-morphisms are equivalent for $j \geq 2$. Thus, up to equivalence, there is no point in mentioning anything beyond $1$-morphisms, except whether two given parallel $1$-morphisms are equivalent.

If you rephrase equivalence of $1$-morphisms as equality, which gives the same result up to equivalence, then all that is left in this definition is a category. Thus one may also say that a **$1$-category** is simply a category.

The point of all this is simply to fill in the general concept of $n$-category; nobody thinks of $1$-categories as a concept in their own right except simply as categories.

The notions of $1$-groupoid and $1$-poset are defined on the same basis.

Revised on March 10, 2012 12:53:11
by Urs Schreiber
(89.204.154.201)