LGAMMA(3)

NAME
lgamma gamma - log gamma function, gamma function

SYNOPSIS
#include <math.h>
extern int
signgam;

double
lgamma(double x);
double
gamma(double x);

DESCRIPTION
Lgamma(x) returns ln|Γ(x)|.

The external integer signgam returns the sign of Γ(x).

Gamma(x) returns Γ(x), with no effect on signgam.

IDIOSYNCRASIES
Do not use the expression “signgam*exp(lgamma(x))” to compute g := Γ(x). Instead use a program like this (in C):

    lg = lgamma(x); g = signgam*exp(lg);

Only after lgamma() has returned can signgam be correct.

For arguments in its range, gamma() is preferred, as for positive arguments it is accurate to within one unit in the last place. Exponentiation of lgamma() will lose up to 10 significant bits.

RETURN VALUES
Gamma() and lgamma() return appropriate values unless an argument is out of range. Overflow will occur for sufficiently large positive values, and non-positive integers. On the VAX, the reserved operator is returned, and errno is set to ERANGE For large non-integer negative values, gamma() will underflow.

SEE ALSO
math(3), infnan(3)

HISTORY
The lgamma function appeared in 4.3BSD. The gamma function appeared in 4.4BSD. The name gamma() was originally dedicated to the lgamma() function, so some old code may no longer be compatible.