Definition
The indicator function of a subset of a set is a function
defined as
The Iverson bracket allows the equivalent notation, to be used instead of
The function is sometimes denoted or or even just . (The Greek letter χ appears because it is the initial letter of the Greek word characteristic.)
Read more about this topic: Indicator Function
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—The Columbia Encyclopedia, Fifth Edition, the first sentence of the article on “life” (based on wording in the First Edition, 1935)
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