__Macro:__**GSL_POSINF**-
This macro contains the IEEE representation of positive infinity,
@math{+\infty}. It is computed from the expression
`+1.0/0.0`

.

__Macro:__**GSL_NEGINF**-
This macro contains the IEEE representation of negative infinity,
@math{-\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 @math{+1} if
`x`is positive infinity, @math{-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.

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