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/* @(#)w_gamma.c 5.1 93/09/24 */
/*
* ====================================================
* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
*
* Developed at SunPro, a Sun Microsystems, Inc. business.
* Permission to use, copy, modify, and distribute this
* software is freely granted, provided that this notice
* is preserved.
* ====================================================
*
*/
/* BUG: FIXME?
According to Linux man pages for tgamma, lgamma, and gamma, the gamma
function was originally defined in BSD as implemented here--the log of the gamma
function. BSD 4.3 changed the name to lgamma, apparently removing gamma. BSD
4.4 re-introduced the gamma name with the more intuitive, without logarithm,
plain gamma function. The C99 standard apparently wanted to avoid a problem
with the poorly-named earlier gamma and used tgamma when adding a plain
gamma function.
So the current gamma is matching an old, bad definition, and not
matching a newer, better definition. */
/*
FUNCTION
<<gamma>>, <<gammaf>>, <<lgamma>>, <<lgammaf>>, <<gamma_r>>, <<gammaf_r>>, <<lgamma_r>>, <<lgammaf_r>>, <<tgamma>>, and <<tgammaf>>---logarithmic and plain gamma functions
INDEX
gamma
INDEX
gammaf
INDEX
lgamma
INDEX
lgammaf
INDEX
gamma_r
INDEX
gammaf_r
INDEX
lgamma_r
INDEX
lgammaf_r
INDEX
tgamma
INDEX
tgammaf
SYNOPSIS
#include <math.h>
double gamma(double <[x]>);
float gammaf(float <[x]>);
double lgamma(double <[x]>);
float lgammaf(float <[x]>);
double gamma_r(double <[x]>, int *<[signgamp]>);
float gammaf_r(float <[x]>, int *<[signgamp]>);
double lgamma_r(double <[x]>, int *<[signgamp]>);
float lgammaf_r(float <[x]>, int *<[signgamp]>);
double tgamma(double <[x]>);
float tgammaf(float <[x]>);
DESCRIPTION
<<gamma>> calculates
@tex
$\mit ln\bigl(\Gamma(x)\bigr)$,
@end tex
the natural logarithm of the gamma function of <[x]>. The gamma function
(<<exp(gamma(<[x]>))>>) is a generalization of factorial, and retains
the property that
@ifnottex
<<exp(gamma(N))>> is equivalent to <<N*exp(gamma(N-1))>>.
@end ifnottex
@tex
$\mit \Gamma(N)\equiv N\times\Gamma(N-1)$.
@end tex
Accordingly, the results of the gamma function itself grow very
quickly. <<gamma>> is defined as
@tex
$\mit ln\bigl(\Gamma(x)\bigr)$ rather than simply $\mit \Gamma(x)$
@end tex
@ifnottex
the natural log of the gamma function, rather than the gamma function
itself,
@end ifnottex
to extend the useful range of results representable.
The sign of the result is returned in the global variable <<signgam>>,
which is declared in math.h.
<<gammaf>> performs the same calculation as <<gamma>>, but uses and
returns <<float>> values.
<<lgamma>> and <<lgammaf>> are alternate names for <<gamma>> and
<<gammaf>>. The use of <<lgamma>> instead of <<gamma>> is a reminder
that these functions compute the log of the gamma function, rather
than the gamma function itself.
The functions <<gamma_r>>, <<gammaf_r>>, <<lgamma_r>>, and
<<lgammaf_r>> are just like <<gamma>>, <<gammaf>>, <<lgamma>>, and
<<lgammaf>>, respectively, but take an additional argument. This
additional argument is a pointer to an integer. This additional
argument is used to return the sign of the result, and the global
variable <<signgam>> is not used. These functions may be used for
reentrant calls (but they will still set the global variable <<errno>>
if an error occurs).
<<tgamma>> and <<tgammaf>> are the "true gamma" functions, returning
@tex
$\mit \Gamma(x)$,
@end tex
the gamma function of <[x]>--without a logarithm.
(They are apparently so named because of the prior existence of the old,
poorly-named <<gamma>> functions which returned the log of gamma up
through BSD 4.2.)
RETURNS
Normally, the computed result is returned.
When <[x]> is a nonpositive integer, <<gamma>> returns <<HUGE_VAL>>
and <<errno>> is set to <<EDOM>>. If the result overflows, <<gamma>>
returns <<HUGE_VAL>> and <<errno>> is set to <<ERANGE>>.
PORTABILITY
Neither <<gamma>> nor <<gammaf>> is ANSI C. It is better not to use either
of these; use <<lgamma>> or <<tgamma>> instead.@*
<<lgamma>>, <<lgammaf>>, <<tgamma>>, and <<tgammaf>> are nominally C standard
in terms of the base return values, although the <[signgam]> global for
<<lgamma>> is not standard.
*/
/* double gamma(double x)
* Return the logarithm of the Gamma function of x.
*
* Method: call gamma_r
*/
#include "fdlibm.h"
#include <reent.h>
#include <errno.h>
#ifndef _DOUBLE_IS_32BITS
#ifdef __STDC__
double gamma(double x)
#else
double gamma(x)
double x;
#endif
{
#ifdef _IEEE_LIBM
return __ieee754_gamma_r(x,&(_REENT_SIGNGAM(_REENT)));
#else
double y;
y = __ieee754_gamma_r(x,&(_REENT_SIGNGAM(_REENT)));
if(_LIB_VERSION == _IEEE_) return y;
if(!finite(y)&&finite(x)) {
#ifndef HUGE_VAL
#define HUGE_VAL inf
double inf = 0.0;
SET_HIGH_WORD(inf,0x7ff00000); /* set inf to infinite */
#endif
if(floor(x)==x&&x<=0.0) {
/* gamma(-integer) or gamma(0) */
errno = EDOM;
} else {
/* gamma(finite) overflow */
errno = ERANGE;
}
return HUGE_VAL;
} else
return y;
#endif
}
#endif /* defined(_DOUBLE_IS_32BITS) */
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