redhat man page for lgamma

LGAMMA(3)						     Linux Programmer's Manual							 LGAMMA(3)

NAME
       lgamma - log gamma function

SYNOPSIS
       #include <math.h>

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

       double lgamma_r(double x, int *signp);
       float lgammaf_r(float x, int *signp);
       long double lgammal_r(long double x, int *signp);

DESCRIPTION
       The Gamma function is defined by

	Gamma(x) = integral from 0 to infinity of t^(x-1) e^-t dt

       It is defined for every real number except for nonpositive integers.  For nonnegative integral m one has

	Gamma(m+1) = m!

       and, more generally, for all x:

	Gamma(x+1) = x * Gamma(x)

       For x < 0.5 one can use

	Gamma(x) * Gamma(1-x) = PI/sin(PI*x)

       The  lgamma()  function	returns  the  natural  logarithm  of  the absolute value of the Gamma function.  The sign of the Gamma function is
       returned in the external integer signgam declared in <math.h>.  It is 1 when the Gamma function is positive or zero, -1 when  it  is  nega-
       tive.

       Since  using  a constant location signgam is not thread-safe, the functions lgamma_r() etc. have been introduced; they return this sign via
       the parameter signp.

       For nonpositive integer values of x, lgamma() returns HUGE_VAL, sets errno to ERANGE and raises the  zero  divide  exception.   (Similarly,
       lgammaf() returns HUGE_VALF and lgammal() returns HUGE_VALL.)

ERRORS
       An  application	wishing  to check for error situations should set errno to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these
       functions. On return, if errno is non-zero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is  non-zero,  an  error
       has occurred.

       ERANGE Invalid argument - nonpositive integer value of x.

CONFORMING TO
       C99, SVID 3, BSD 4.3

SEE ALSO
       tgamma(3)

								    2002-08-10								 LGAMMA(3)