# In what cases does a Yoneda-like embedding preserve monoidal structure?

What kinds of Yoneda-like situations induce an embedding that preserves the tensor product for some arbitrary monoidal category?

The cases where the monoidal product is given by a limit or colimit give this immediately for the usual Yoneda embedding, but this breaks down for "real" monoidal categories like $(Vect, \otimes)$.

Are there $V$-enriched cases where the generalised embedding

$$Y : C \to V^{C^{op}}$$

does preserve the tensor product for interesting monoidal categories $C$?

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Day showed that, for suitable V, any monoidal structure on a (V-)functor category $[C^{\mathrm{op}}, V]$ is essentially determined by its restriction to the representables as $$F \otimes G = \int^{A,B} F A \otimes G B \otimes P(A,B,-)$$ where $P(A,B,-) = C(-, A) \otimes C(-, B)$ is a profunctor $C \otimes C \otimes C^{\mathrm{op}} \to V$. P (together with a unit and the usual structural isos) is said to endow C with a promonoidal structure.

If C is already a monoidal V-category, then there is a canonical promonoidal structure on it given by $$C(-, A) \otimes C(-, B) = C(-, A \otimes B)$$ In that case, the Yoneda embedding is strong monoidal by definition. In fact it is the unit for the monoidal cocompletion of C.

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Could you give a reference to the paper of Day? – Harry Gindi Aug 20 '10 at 17:42
Yes, sorry, it's in his 1970 thesis, to be found at math.mq.edu.au/~street/Day.pub.html – Finn Lawler Aug 20 '10 at 18:10
I think you mean any closed monoidal structure on a functor category? – Mike Shulman Aug 21 '10 at 2:47
Yes, I should also have said that this assumes that the tensor on both V and [C^op, V] is cocontinuous in both variables. – Finn Lawler Aug 21 '10 at 19:58