Aligning equations in a cases environment
I think that you want to use \left\{...\right.
around your equation. To be able to align your equations inside these you can use the split
environment from the amsmath package.
EDIT
In the comments both above and below the consensus is that the aligned
environment is superior to split
in this situation. Perhaps I am missing it but I can't see the difference in this example, however, as people more knowledgeable than I are advocating using the aligned
environment there must be cases where it does matter.
Here is a comparison of the two environments for this example:
This produces:
\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator\vol{\text{vol}}
\begin{document}
\[\left\{
\begin{aligned}
\pi_n (i) &= {n^2}/{\vol(G_n)} \cr
\pi_n (j) &= {n}/{\vol(G_n)} \cr
q^{(n)}_{ij} &= {1}/{n^2 } \cr
q^{(n)}_{ji} &= {1}/{n}
\end{aligned}\right.\tag{\textbf{aligned}}
\]
\[\left\{
\begin{split}
\pi_n (i) &= {n^2}/{\vol(G_n)} \cr
\pi_n (j) &= {n}/{\vol(G_n)} \cr
q^{(n)}_{ij} &= {1}/{n^2 } \cr
q^{(n)}_{ji} &= {1}/{n}
\end{split}\right.\tag{\textbf{split}}
\]
\end{document}