lgamma - log gamma function
#include <math.h>
double lgamma(double x);
The lgamma() function returns the log of the absolute
value of the Gamma function. The sign of the Gamma func
tion is returned in the external integer signgam.
For negative integer values of x, lgamma() returns
HUGE_VAL and errno is set to ERANGE.
ERANGE Invalid argument - negative integer value of x.
SVID 3, BSD 4.3
infnan(3)