The cases environment renders multiple lines with an extensible left curly-brace. It can be used for piecewise-defined functions. For this to work, you must have \usepackage{amsmath} in the preamble.

LaTeX markup... ...results in: used for:
f(n) = \begin{cases} 1/(b-a) &\mbox{if } n \equiv 0 \\
(3n +1)/2 & \mbox{if } n \equiv 1 \end{cases} \pmod{2}.
function defined piecewise
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