small category

basic constructions:

strong axioms

further

A category is **small** if it has a small set of objects and a small set of morphisms.

In other words, a small category is an internal category in the category Set.

A category which is not small is called large.

Small categories are free of some of the subtleties that apply to large categories.

A category is said to be **essentially small** if it is equivalent to a small category. Assuming the axiom of choice, this is the same as saying that it has a small skeleton, or equivalently that it is locally small and has a small number of isomorphism classes of objects.

A **small category structure** on a locally small category $C$ is an essentially surjective functor from a set (as a discrete category) to $C$. A category is essentially small iff it is locally small and has a small category structure; unlike the previous paragraph, this result does not require the axiom of choice.

If Grothendieck universes are being used, then for $U$ a fixed Grothendieck universe, a category $C$ is **$U$-small** if its collection of objects and collection of morphisms are both elements of $U$. $C$ is **essentially $U$-small** if there is a bijection from its set of morphisms to an element of $U$ (the same for the set of objects follows); this condition is non-evil.

So let $U\Set$ be the category of $U$-small sets. Then

- a $U$-category (a locally U-small)-category is a category enriched over $U Set$;
- a $U$-small category is a category internal to $U Set$.

A category is **$U$-moderate** if its set of objects and set of morphisms are both subsets of $U$. However, some categories (such as the category of $U$-moderate categories!) are larger yet.

Last revised on July 28, 2018 at 07:39:30. See the history of this page for a list of all contributions to it.