Modules Over Commutative Monoid – Symmetric Monoidal Category

ct.category-theorymonoidal-categoriesreference-request

Let $\left(\cal{C},\otimes ,I\right)$ be a symmetric monoidal category (not necessarily closed) and $A$ a commutative monoid in $\cal{C}$. In his DAG III (page 95), Lurie writes:

In many cases, the category $\cal{M}od_A\left(\cal{C}\right)$ of $A$-modules in
$\cal{C}$ inherits the structure of a symmetric monoidal category with respect to the relative tensor product over $A$.

Where can I find conditions, details and proofs of this (seemingly) elementary fact? (I didn't find it in the book I searched – MacLane, Barr & Wells, or in books about operads etc.)

I also need the facts that "extension of scalars" $-\otimes A$ is the left adjoint of the forgetful functor and it commutes with the tensor product. And the case when $\otimes$ distriute over the coproduct (or the bi-product).

I think I can prove most of these facts, but I want to be sure about them, and it would also be much easier to refer to them.

Best Answer

One needs that $C$ is cocomplete and that $\otimes$ preserves colimits in each variable (and then $\mathrm{Mod}_C(A)$ will have the corresponding properties). More precisely, you only need that $C$ has reflexive coequalizers and that $\otimes$ preserves them in each variable. This has been known for decades, but the first clear write-up of this, at least I know of, is Florian Marty's thesis. You can also find a discussion on this in my thesis, Section 4.1 (and Chapter 6 for the issue on reflexive coequalizers). See also MO/114457 for a discussion of the internal homs by Todd Trimble.

Edit. Here are some other references, which even discuss the case where $- \otimes A$ is replaced by a (suitable) symmetric monoidal monad:

H. Lindner. Commutative monads. In Deuxiéme colloque sur l'algébre des catégories. Amiens-1975. Résumés des conférences, pages 283-288. Cahiers de topologie et géométrie différentielle catégoriques, tome 16, nr. 3, 1975.

R. Guitart. Tenseurs et machines. Cahiers de Topologie et Géométrie Différentielle Catégoriques, 21(1):5-62, 1980.

A. Kock. Closed categories generated by commutative monads. Journal of the Australian Mathematical Society, 12(04):405-424, 1971.

G. J. Seal. Tensors, monads and actions. Theory Appl. Categ., 28:No. 15, 403-433, 2013.

If I recall correctly, some of these references restrict the monads in such a way that they are of the form $- \otimes A$ anyway.

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