# Indicator function

A function indicating membership in a set, say $LaTeX: A$. Such functions are usually denoted $LaTeX: \chi_A$ or $LaTeX: 1_A$ and are evaluated as
$LaTeX: \chi_A (x) = 1_A = \left\{ \begin{array}{cl} 1, & x \in A \\ 0, & x \not\in A. \end{array} \right.$