Several lines in equation

Question

I would like to get in LaTeX what the following code produces on Math.SE:

$$\pi(X,Y)\to HoM(X,Y)\\\  
[f]\mapsto[RQf]$$

And if I wanted to have an equation number to the right of the two lines, how would I do that?

Answer 1

For example (Please observe changing of the meaning of HoM. I am assuming that it is your intention):

\documentclass{article}
\usepackage{amsmath}

\DeclareMathOperator{\HoM}{Hom}
\begin{document}


The first bijection is induced...
\begin{equation}
\begin{aligned}
\pi(X,Y)&\to\HoM(X,Y)\\
[f]&\mapsto[RQf]
\end{aligned}
\end{equation}
if $X$ is cofibrant and $Y$ fibrant.

\end{document}