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Mathematica® Teacher's Edition (2002)

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NumberQ

FilledSmallSquare NumberQ[expr] gives True if expr is a number, and False otherwise.

FilledSmallSquare NumberQ[expr] returns False unless expr is manifestly a number (i.e., has head Complex, Integer, Rational or Real).
FilledSmallSquare NumberQ[Infinity] gives False.
FilledSmallSquare NumberQ[Overflow[ ]] and NumberQ[Underflow[ ]] give True.
FilledSmallSquare See The Mathematica Book on the web: Section 2.3.5 and Section 3.1.1.
FilledSmallSquare See also: NumericQ, IntegerQ, TrueQ, Complexes.