[Tex/LaTex] How to write a two line function definition? (With domain/codomain on one line, and element/mapsto/element on the other.)

Question

How can I write this to look as neat as this:

Edit: What I have tried so far

\begin{eqnarray*}
M_f : \pi_1(S \backslash \{y_1, \cdots, y_n\}, y) &\rightarrow& \text{Bij}(f^{-1}(y)) \\
\gamma &\mapsto& M_f(\gamma) = \sigma_\gamma^{-1}
\end{eqnarray*}

which yields

This does not align nicely like the upper formula.

(BTW, how to get the better looking \pi in the first picture?)

Answer 1

Here are two possibilities (inline or not). Don't use eqnarray, which has bad horizontal spacing around the alignment points, and replace it with align* or aligned. Also, for set difference, use \(small)setminus:

\documentclass{article}
\usepackage{amsmath, amssymb}
\DeclareMathOperator\Bij{Bij}

\begin{document}

\begin{align*}
  M_f \colon \pi_1(S \smallsetminus \{y_1, \dots, y_n\}, y) & \longrightarrow \Bij(f^{-1}(y)) \\
  \gamma & \longmapsto M_f(\gamma) = \sigma_\gamma^{-1}
\end{align*}

Or this:\medskip

Let $ S' =S \smallsetminus \{y_1, \dots, y_n\} $. Define
$ \begin{aligned}[t]
M_f \colon \pi_1(S', y) &\longrightarrow \Bij(f^{-1}(y)) \\
\gamma &\longmapsto M_f(\gamma) = \sigma_\gamma^{-1}
\end{aligned} $
\end{document}