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Showing content from https://cplusplus.com/reference/cmath/lgamma/ below:

function

lgamma

     double lgamma  (double x);      float lgammaf (float x);long double lgammal (long double x);

     double lgamma (double x);      float lgamma (float x);long double lgamma (long double x);     double lgamma (T x);           // additional overloads for integral types

Compute log-gamma function

Returns the natural logarithm of the absolute value of the gamma function of x.

Header

<tgmath.h>

provides a type-generic macro version of this function.

Additional overloads

are provided in this header (

<cmath>

) for the

integral types

: These overloads effectively cast

to a

double

before calculations (defined for

being any

integral type

Parameters

x: Parameter for the log-gamma function.

Return Value Log-gamma function of x.
If x is too large, an overflow range error occurs.
If x is zero or a negative integer for which the function is asymptotic, it may cause a pole error (depending on implementation).

If an overflow range error occurs:

- And math_errhandling has MATH_ERRNO set: the global variable errno is set to ERANGE.
- And math_errhandling has MATH_ERREXCEPT set: FE_OVERFLOW is raised.

If a pole error occurs:

- And math_errhandling has MATH_ERRNO set: the global variable errno is set to ERANGE.
- And math_errhandling has MATH_ERREXCEPT set: FE_DIVBYZERO is raised.

Example

/* lgamma example */
#include <stdio.h>      /* printf */
#include <math.h>       /* lgamma */

int main ()
{
  double param, result;
  param = 0.5;
  result = lgamma (param);
  printf ("lgamma(%f) = %f\n", param, result );
  return 0;
}

Output:

lgamma (0.500000) = 0.572365