Why do MathJax piecewise function (case blocks) not render correctly?

Question

When using MathJax I thought that \begin{cases}...\end{cases} would have each case printed on a separate line.

Instead, the separate cases all all listed on the same line, when I look at it.

How do we fix this?

\sigma(n) =
\begin{cases}
n/2,  & \text{if $n$ is even} \\   
3n+1, & \text{if $n$ is odd}  \\
\end{cases}

$ \sigma(n) = \begin{cases} n/2, & \text{if $n$ is even} \\ 3n+1, & \text{if $n$ is odd} \\ \end{cases} $