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- 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.^{6}

- 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.

Note that the C99 standard only requires the
system `isinf`

function to return a non-zero value, without the
sign of the infinity. The implementation in some earlier versions of
GSL used the system `isinf`

function and may have this behavior
on some platforms. Therefore, it is advisable to test the sign of
`x` separately, if needed, rather than relying the sign of the
return value from `gsl_isinf()`

.