ACIT4330-Page/content/Definitions/Functions/Characteristic Function.md

\varkappa_{y} \in \Pi_{X} \, \{ 0, \, 1 \}, so \varkappa_{y} : X \to \{ 0, \, 1 \} defined as: $$\varkappa_{y}(x) = \begin{cases} 1, & \text{if}\ x \in Y\ 0, & \text{if}\ x \notin Y \end{cases}$$