There are a few abstract "numbers" defined by the IEEE FPstandard
for exmaple:
Signed zero
Denormalized number
Signed infinity
Not a Number
IEEE FloatingPoint Values
Name Quantity Exponent Significand
Not a Number NaN E = Emax + 1 sig not equal 0
Not a Number (NaN) results from an operation involving one or more invalid operands. For instance 0/0 and SQRT(1) result in NaN. In general, an operation involving a NaN produces another NaN. Because the fraction of a NaN is unspecified, there are many possible NaNs depending on what the significand is. In this case, any value will do.
Tim
