Let mathcalVmathcalV be a reasonably nice category — I'm interested in the case when mathcalVmathcalV is mathbbKmathbbK-linear for some field mathbbKmathbbK, abelian, and has all products and coproducts (hence all limits and colimits, as it is abelian), but I don't mind if you demand similar or weaker properties — with a reasonably nice monoidal structure otimesotimes. Recall that a (counital coassiciative) coalgebra in mathcalVmathcalV is an object AinmathcalVAinmathcalV along with maps Ato1Ato1 and AtoAotimesAAtoAotimesA that are coassiciative and counital in the sense that certain diagrams commute. The notion of homomorphism of coalgebras is obvious.
When mathcalV=textVECTmathcalV=textVECT is the category of vector spaces, then coalgebras satisfy a particular fundamental property that makes them essentially easy. Namely, any coalgebra is textVECTtextVECT is the (vector space) sum of its finite-dimensional subcoalgebras. On the other hand, the corresponding statement in textVECTrmoptextVECTrmop fails: it is not true that every algebra in textVECTtextVECT is a pullback of its finite-dimensional quotient algebras. This is in spite of the fact that for many purposes textVECTtextVECT and textVECTrmoptextVECTrmop are equally nice categories.
For a general sufficiently nice category mathcalVmathcalV, I should replace the word "sum" by "limit" and I should replace "finite-dimensional" by "dualizable". All together, my question is:
For which sufficiently nice monoidal categories is it true that every coalgebra object is a limit of its dualizable subcoalgebra objects?
This is, of course, an open ended question. The very best would be some necessary and sufficient conditions that are easier to check, but that's probably too hard: natural (and naturally occurring) easily-checked sufficient conditions would suffice.
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