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It is a well-known fact that on categories admitting all finite coproducts one can define a monoidal structure where the monoidal product is exactly the coproduct and the monoidal unit is the initial object.

For coproducts there is nothing that prohibits me from working with a countable (or even uncountable) number of objects. Now I was wondering if there exists any general theory on how this can be generalized to the monoidal context? (In general I have barely found anything on the extension of monoidal products to an infinite number of objects.)

This question popped up while I was studying monoidal posets (see Seven Sketches in Compositionality by Spivak & Fong: http://math.mit.edu/~dspivak/teaching/sp18/7Sketches.pdf). On one hand I am interested in this question from an educational point of view because I would in general like to know how the concepts from monoidal category theory generalize to ''infinite products''. But on the other hand I am also interested from a professional point of view because not all operations I consider come from coproducts (e.g. the addition on $\mathbb{R}^+$) and hence I cannot resort to a description in those terms.

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  • $\begingroup$ You can construct infinitary coproducts from finite coproducts and directed colimits in a straightforward fashion (it is say, in MacLane's book). My suggestion is to define infinitary monoidal products in the same way. $\endgroup$ Commented Aug 28, 2020 at 12:20
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    $\begingroup$ Does this answer your question? $\infty$-ary tensor product on a category $\endgroup$
    – varkor
    Commented Aug 28, 2020 at 12:34
  • $\begingroup$ This indeed gives an idea of what the infinitary construction would be (at least in abstract terms). I assume that the definition of infinitary (lax) monoidal functors will also be a suitable modification of the finitary definition from Leinster's Higher operads, higher categories? $\endgroup$
    – NDewolf
    Commented Aug 28, 2020 at 12:44
  • $\begingroup$ Yes, that's right. $\endgroup$
    – varkor
    Commented Aug 28, 2020 at 14:32
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    $\begingroup$ This paper might be of interest. $\endgroup$
    – Arnaud D.
    Commented Aug 30, 2020 at 21:36

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