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RedHat 9 (Linux i386) - man page for lgamma (redhat section 3)

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

lgamma - log gamma function
#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);
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 negative. Since using a constant location signgam is not thread-safe, the functions lgamma_r() etc. have been intro- duced; 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.)
An application wishing to check for error situations should set errno to zero and call feclearex- cept(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.
C99, SVID 3, BSD 4.3
tgamma(3) 2002-08-10 LGAMMA(3)

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