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Enriched categories, topoi, abelian categories, monoidal categories, homological algebra.

11 votes
Accepted

Has the Isbell–Freyd criterion ever been used to check that a category is concretisable?

An inverse category can be defined as a category where every $f$ admits a unique regular inverse, i.e. a map $g$ such that $fgf=f$ and $gfg=g$. In [1], Kastl proves that any locally small inverse cate …
Martti Karvonen's user avatar
11 votes
Accepted

Naturality up to (inner) automorphism?

Assuming the inner automorphisms you get are assumed to satisfy some further coherency conditions, your morphisms should amount to pseudonatural transformations. This requires thinking of the category …
Martti Karvonen's user avatar
15 votes
1 answer
406 views

Surprising invertibility results

There are results in category theory that imply that some morphism is invertible when a priori one might not have expected it. For instance, Given a monoidal natural transformation $\tau$ between str …
2 votes
Accepted

Strict 2-Category with Lax Tensor?

This suggests that the monoidal product $\otimes\colon \mathbf{C}^2\to \mathbf{C}$ is a lax functor (see e.g. here or section 4.1 here) instead of a strict 2-functor. If so, this determines which coh …
Martti Karvonen's user avatar
10 votes
Accepted

Why does the 2-category of groups have (some, strict) coinserters but not (strict) inserters?

One viewpoint goes as follows: the 2-categorical structure on groups can be seen as coming from inner automorphisms, so that a 2-cell is given by an inner automorphism that translates one map to the o …
Martti Karvonen's user avatar