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

11 votes
2 answers
1k views

What is the category of covariant and contravariant functors?

Let $\bf Cat'$ be the category that has as objects small categories $A, B...$, and as arrows functors $F:A\to B$ that are either covariant or contravariant. The identity on $A\in\bf Cat'$ is the usual …
Claudio Pisani's user avatar
1 vote
1 answer
126 views

Products in discrete fibrations

Let $B$ be a category with products and let $F:A\to B$ be a discrete opfibration. Let $F^*:B\to \bf Set$ be the functor corresponding to $F$ under the Grothendieck correspondence. The following propos …
Claudio Pisani's user avatar
6 votes
0 answers
212 views

Is the category of projections interesting?

Let $C$ be a category and let $C'$ be the wide subcategory whose maps are projections, that is maps in $C$ which belong to some limiting cone (over a discrete base). Since limiting cones compose, $C'$ …
Claudio Pisani's user avatar
6 votes
1 answer
332 views

When is an object determined by the number of maps from the other objects?

Let $C$ be a category with finite hom-sets. Suppose that $X$ and $Y$ are objects in $C$ such that $C(Z,X)\cong C(Z,Y)$ for any Z (with no naturality condition). For which categories $C$ does it follow …
Claudio Pisani's user avatar