## Infinities and Not-a-number

Macro: GSL_POSINF
  This macro contains the IEEE representation of positive infinity, +\infty. It is computed from the expression `+1.0/0.0`.

Macro: GSL_NEGINF
  This macro contains the IEEE representation of negative infinity, -\infty. It is computed from the expression `-1.0/0.0`.

Macro: GSL_NAN
  This macro contains the IEEE representation of the Not-a-Number symbol, `NaN`. It is computed from the ratio `0.0/0.0`.

Function: int gsl_isnan (const double x)
  This function returns 1 if x is not-a-number.

Function: int gsl_isinf (const double x)
  This function returns +1 if x is positive infinity, -1 if x is negative infinity and 0 otherwise.

Function: int gsl_finite (const double x)
  This function returns 1 if x is a real number, and 0 if it is infinite or not-a-number.