I'm using this
\newcommand\Underset[2]{\underset{\textstyle #1}{#2}}
so that
\Underset{f \mapsto f\circ \partial_{n} }{\partial^{*}_{n}:Hom(G_{n-1},M)\to Hom(G_{n},M)}
wouldn't minimize the function under those sets, but now how do I align f
under Hom
and the row \mapsto
under the row \to
?