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gammln.cc
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1 // $Id: gammln.cc,v 1.3 2003/08/13 20:00:12 garren Exp $
2 // -*- C++ -*-
3 //
4 // -----------------------------------------------------------------------
5 // HEP Random
6 // --- HepStat::gammln ---
7 // method implementation file
8 // -----------------------------------------------------------------------
9 
10 // =======================================================================
11 // M. Fischler - moved the gammln from RandPoisson to here. 01/26/00
12 // =======================================================================
13 
14 #include "CLHEP/Random/Stat.h"
15 #include "CLHEP/Random/defs.h"
16 #include <cmath>
17 
18 namespace CLHEP {
19 
20 double HepStat::gammln(double xx) {
21 
22 // Returns the value ln(Gamma(xx) for xx > 0. Full accuracy is obtained for
23 // xx > 1. For 0 < xx < 1. the reflection formula (6.1.4) can be used first.
24 // (Adapted from Numerical Recipes in C. Relative to that routine, this
25 // subtracts one from x at the very start, and in exchange does not have to
26 // divide ser by x at the end. The results are formally equal, and practically
27 // indistinguishable.)
28 
29  static double cof[6] = {76.18009172947146,-86.50532032941677,
30  24.01409824083091, -1.231739572450155,
31  0.1208650973866179e-2, -0.5395239384953e-5};
32  int j;
33  double x = xx - 1.0;
34  double tmp = x + 5.5;
35  tmp -= (x + 0.5) * std::log(tmp);
36  double ser = 1.000000000190015;
37 
38  for ( j = 0; j <= 5; j++ ) {
39  x += 1.0;
40  ser += cof[j]/x;
41  }
42  return -tmp + std::log(2.5066282746310005*ser);
43 }
44 
45 } // namespace CLHEP
46 
47 
CLHEP
Definition: ClhepVersion.h:13
j
long j
Definition: JamesRandomSeeding.txt:28
x
any side effects of that construction would occur twice The semantics of throw x
Definition: whyZMthrowRethrows.txt:37
CLHEP::HepStat::gammln
static double gammln(double x)
Definition: gammln.cc:20