nLab exact 2-category

Idea

The notion of exact 2-catory is the analog in 2-category theory of the notion of exact category in category theory.

Definition

A regular 1-category is exact when every congruence has a kernel. The definition for 2-categories is analogous.

Definition

Let $-1\lt n\le 2$ be directed (see (n,r)-category). A 2-category is $n$-exact if it is regular and every (n,r)-congruence is a kernel.

Remark

Recall (from regular 2-category) that a 1-category is regular as a 1-category iff it is regular as a homwise-discrete 2-category. However, a regular 1-category is exact as a 1-category precisely when it is 1-exact as a 2-category. Since general 2-congruences in a 1-category are internal categories, they will obviously not all be kernels. This is why we add a prefix to “exact:” the best notion for 2-categories, which we call “2-exact,” is not a conservative extension of the established meaning of “exact” for 1-categories.

Likewise, it is unreasonable to expect a (2,1)-category to be any more than (2,1)-exact, or a (1,2)-category to be any more than (1,2)-exact.

Examples

• Cat is 2-exact. Likewise, Gpd is (2,1)-exact, Pos is (1,2)-exact, and of course Set is 1-exact.

• Every regular (0,1)-category (that is, every meet-semilattice) is (0,1)-exact, and in fact even 2-exact, since there are no nontrivial congruences of any sort in a poset.

• If $K$ is 2-exact, then by the classification of congruences, $gpd(K)$ is (2,1)-exact, $pos(K)$ is (1,2)-exact, $disc(K)$ is 1-exact, and $Sub(1)$ is (0,1)-exact.

• If $K$ is $n$-exact, then so is $K^{co}$, by the remarks about opposite 2-congruences. For 1-categories and (2,1)-categories, of course, this is contentless, but for 2-categories and (1,2)-categories it is contentful.

Properties

Exact completion

See at 2-congruence the section Exactness.

References

The definition is originally due to

• Michael Shulman, exact 2-category

based on

• Ross Street, StreetCBS