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Characteristic functions

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A characteristic function (or indicator function)

returns 1 if has the defining property of a given set or 0 otherwise.

For example, the characteristic function of the primes yields 1 only when is prime (see A010051). Another example is the characteristic function of squarefree numbers (see A008966).

See Index to OEIS: Section Ch for a thorough listing of characteristic function sequences in the OEIS.