Computing (FOLDOC) dictionary
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for the result of a numerical operation which cannot return a
valid number value. A NaN can result from multiplying an
infinity by a zero, or from subtracting one infinity from
another [what else?].
NaN is encoded as a special bit pattern
is used to signal error returns where other mechanisms are not
errors to propagate through a calculation.
Similar bit patterns represent positive and negative
[ANSI/IEEE Std 754-1985].