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13 votes
3 answers
2k views

How cavalier can I be when demanding a category have direct sums?

In my meaning, a direct sum in a category should really be called a "biproduct". If $X,Y$ are objects, then a direct sum $X \oplus Y$ is an object $Z$ along with isomorphisms $\hom(Z,A) = \hom(X,A) \ …
Theo Johnson-Freyd's user avatar
9 votes
2 answers
2k views

What are examples of cogenerators in R-mod?

Fill in the blank, please :) Let $\mathcal C$ be a complete and cocomplete abelian category. A generator in $\mathcal C$ is an object $X \in \mathcal C$ such that every object $Y \in \mathcal C$ …
Theo Johnson-Freyd's user avatar
8 votes
2 answers
771 views

When is/isn't the monoidal unit compact projective?

I am interested in developing intuition for when the monoidal unit in a closed monoidal abelian category is or isn't compact projective. As such, my question is not looking for a yes/no answer, but r …
Theo Johnson-Freyd's user avatar
9 votes
1 answer
594 views

In a closed monoidal abelian category, are the compact projectives a monoidal subcategory?

Question: In a closed monoidal abelian category such that the unit object is compact projective, must the tensor product of compact projective objects be compact projective? Recall that an object …
Theo Johnson-Freyd's user avatar