# Adjunction of functors between monoidal categories induces adjunction between their categories of internal monoids

Let $(C,\odot), (D,\square)$ be monoidal categories. If $F \colon D \to C$ is left adjoint to $U \colon C \to D$, where $F$ is a monoidal functor ($U$ does not need to be), is it the case that the induced functors $F' \colon Mon(D,\square) \to Mon(C, \odot)$ and $U' \colon Mon(C,\odot) \to Mon(D,\square)$ are adjoint? (EDIT: $F'$ is induced by $F$ in the natural way since $F$ is monoidal; $U'$ is induced by $U$ in the sense that the square with $U, U'$ and the forgetful functors from internal monoids to their monoidal categories commute).
Here's my motivation for the question: if $(D, \square)=(Set, \times)$ (where $\times$ is the cartesian product), $(C, \odot)=(Vect_{\mathbb{K}}, \otimes)$ (where $\otimes$ is the usual tensor product), $U$ is the forgetful functor which takes vector spaces to their underlying sets and linear maps to their underlying functions and $F$ takes a set to a vector space which basis is that set and functions to uniquely determined linear maps, you also have functors $U_V \colon Mon(Vect_{\mathbb{K}}, \otimes) \to Vect_{\mathbb{K}}, U_S \colon Mon(Set, \times) \to Set$ (the usual forgetful functors from associative unital algebras to vector spaces and from monoids to sets, respectively), which by Theorem 2 (Construction of free monoids) in CWM have left adjoints $F_V, F_S$.
It's elementary that $U U_V = U_S U'$. If I could prove that $U'$ is right adjoint to $F'$, then $U_S U'$ would be right adjoint to $F' F_S$. Since $U U_V$ is already a right adjoint to $F_V F$, we would have a natural isomorphism $F_V F \cong F' F_S$, which I wish to prove (without messy calculations).
• Is $\times$ necessarily the categorical product ? And how does $U$ induce $U'$ if it is not monoidal ? – Max Aug 13 '18 at 21:09
• @Max I apologize for the bad choice there. It doesn't need to be the categorical product, but for my purposes it suffices. Also you're absolutely right, $U$ can't induce $U'$ in a direct way if it's not monoidal. So let's say that $U$ induces $U'$ in the sense that it makes the square with right adjoints and $U'$ to commute. – Hilario Fernandes Aug 13 '18 at 21:36
• @HilarioFernandes: FYI, \square ($\square$) is a common symbol to use for monoidal operations when you can't use $\otimes$. Some other options are \odot and \star ($\odot$ and $\star$). – Hurkyl Aug 13 '18 at 21:55