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.\"     from: @(#)lgamma.3	6.6 (Berkeley) 12/3/92
.\"
.Dd December 3, 1992
.Dt LGAMMA 3
.Os
.Sh NAME
.Nm lgamma ,
.Nm lgammaf
.Nd log gamma functions
.Sh SYNOPSIS
.Fd #include <math.h>
.Ft extern int
.Fa signgam ;
.sp
.Ft double
.Fn lgamma "double x"
.Ft float
.Fn lgammaf "float x"
.Sh DESCRIPTION
.Fn lgamma x
.if t \{\
returns ln\||\(*G(x)| where
.Bd -unfilled -offset indent
\(*G(x) = \(is\d\s8\z0\s10\u\u\s8\(if\s10\d t\u\s8x\-1\s10\d e\u\s8\-t\s10\d dt	for x > 0 and
.br
\(*G(x) = \(*p/(\(*G(1\-x)\|sin(\(*px))	for x < 1.
.Ed
.\}
.if n \
returns ln\||\(*G(x)|.
.Pp
The external integer
.Fa signgam
returns the sign of \(*G(x).
The
.Fn lgammaf
function is a single precision version of
.Fn lgamma .
.Sh IDIOSYNCRASIES
Do not use the expression
.Sq Li signgam\(**exp(lgamma(x))
to compute g := \(*G(x).
Instead use a program like this (in C):
.Bd -literal -offset indent
lg = lgamma(x); g = signgam\(**exp(lg);
.Ed
.Pp
Only after
.Fn lgamma
has returned can signgam be correct.
.Sh RETURN VALUES
.Fn lgamma
returns appropriate values unless an argument is out of range.
Overflow will occur for sufficiently large positive values, and
non-positive integers.
On the
.Tn VAX ,
the reserved operator is returned,
and
.Va errno
is set to
.Er ERANGE .
.Sh SEE ALSO
.Xr infnan 3 ,
.Xr math 3
.Sh HISTORY
The
.Fn lgamma
function appeared in
.Bx 4.3 .