how to align this

Question

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?

Answer 1

I'm not sure what you are looking for but, well, like this?

\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator{\Hom}{Hom}
\begin{document}
\begin{align*}
\partial^{*}_{n}:\Hom(G_{n-1},M)&\to\Hom(G_{n},M)\\
f &\mapsto f\circ \partial_{n}
\end{align*}
\end{document}