indicator function
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function that returns 1 if an element is present in a specified subset and 0 if absent; naturally isomorphic with a set's subsets
~14 min read
Encyclopedic overview
A three-dimensional plot of an indicator function, shown over a square two-dimensional domain (set X): the "raised" portion overlays those two-dimensional points which are members of the "indicated" subset (A). 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 A is a subset of some set X, then the indicator function of A is the function
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Excerpted from Wikipedia’s “indicator function” article, available under the CC BY-SA 4.0 licence.