Connected monads weakly preserve products

Abstract

If $F$ is a (not necessarily associative) monad on $Set$, then the natural transformation $F(A\times B)\to F(A)\times F(B)$ is surjective if and only if $F(1)=1$. Specializing $F$ to $F_{\mathcal{V}}$, the free algebra functor for a variety $\mathcal{V}$, this result generalizes and clarifies an observation by Dent, Kearnes and Szendrei.