According to Wikipedia (https://en.wikipedia.org/wiki/Indicator_function, "Indicator function", 11-Apr-2026): "In mathematics, an indicator function or a characteristic function of a subset of a set is a function that maps elements of the subset to one, and all other elements to zero. That is, if is a subset of some set , then the indicator function of is the function A defined by A ( x ) = 1 if , and A ( x ) = 0 otherwise."
See also definition in [Lang2] p. 3: "The characteristic function of a subset S' of S is the function such that (x) = 1 if ' and (x) = 0 if '".