bsd man page for lgamma

LGAMMA(3M)																LGAMMA(3M)

NAME
       lgamma - log gamma function

SYNOPSIS
       #include <math.h>

       double lgamma(x)
       double x;

DESCRIPTION
		  _
       Lgamma returns ln|| (x)|.
							 _
       The external integer signgam returns the sign of | (x) .

IDIOSYNCRASIES
								 _
       Do not use the expression signgam*exp(lgamma(x)) to compute g := | (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.  Note too that | (x) must overflow when x is large enough, underflow when -x is
       large enough, and spawn a division by zero when x is a nonpositive integer.
										  _
       Only in the UNIX m_th library for C was the na_e gamma ever attached to ln| .  Elsewhere, for instance in IBM's FORTRAN library,  the  name
       GAMMA belongs to |  and the name ALGAMA to ln|  in single precision; in double the names are DGAMMA and DLGAMA.	Why should C be different?
									      _
       Archaeological records suggest that C's gamma originally delivered ln(| (|x|)).	Later, the program gamma was changed to cope with negative
       arguments x in a more conventional way, but the documentation did _ot reflect that change correctly.  The most recent change corrects inac-
       curate  values  when  x	is  almost a negative integer, and lets | (x) be computed without conditional expressions.  Programmers should not
       assume that lgamma has settled down.

       At some time in the future, the name gamma will be rehabilitated and used for the gamma function, just as is done in FORTRAN.   The  reason
       for  this is not so much compatibility with FORTRAN as a desire to achieve greater speed for smaller values of |x| and greater accuracy for
       larger values.

       Meanwhile, programmers who have to use the name gamma in its former sense, for what is now lgamma, have two choices:

       1) Use the old math library, libom.

       2) Add the following program to your others:
	  #include <math.h>
	  double gamma(x)
	  double x;
	  {
		 return (lgamma(x));
	  }

DIAGNOSTICS
       The reserved operand is returned on a VAX for negative integer arguments, errno is set to ERANGE; for very large arguments  over/underflows
       will occur inside the lgamma routine.

SEE ALSO
       math(3M), infnan(3M)

4.3 Berkeley Distribution					   May 12, 1986 							LGAMMA(3M)