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Let $k$ be a field, and let $\mathcal{C}$ be a locally finitely presentable $k$-linear categories. The Deligne-Kelly tensor product $\mathcal{C}\boxtimes\mathcal{C}$ is the 2-universal locally finitely presentable $k$-linear category through which every bilinear functor out of $\mathcal{C}\times\mathcal{C}$ that is right exact in both variables factors.

If $\mathcal{C}$ has a monoidal structure $\otimes$ right exact in both variables, one can therefore consider the canonical right exact functor $T:\mathcal{C}\boxtimes \mathcal{C}\rightarrow \mathcal{C}$ induced by $\otimes$.

If $\otimes$ preserves compact objects and every compact object has both a left and a right dual in $\mathcal{C}$, is it true that $T$ has a left adjoint? (In fact, it is enough to prove that $T$ is left exact.)

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  • $\begingroup$ I think the condition you're asking for implies instead that $T$ has a colimit preserving right adjoint. If $H$ is an Hopf algebra, then the category of $H$-modules does not satisfy your conditions, yet the tensor product has a left adjoint (induction along the coproduct). On the other hand, the category of $H$-comodules does satisfy your assumption, but then (I'm not entirely sure) I think the tensor product won't have a left adjoint in general $\endgroup$
    – Adrien
    Commented Apr 24, 2022 at 10:32

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