erfQ.cc

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// $Id:$
// -*- C++ -*-
//
// -----------------------------------------------------------------------
//                             HEP Random
//                          --- erfQ ---
//                      method implementation file
// -----------------------------------------------------------------------
 
// Contains methods that do not depend on large tables.
//
// erfQ       (double x)
 
// =======================================================================
// M Fischler     - Created 1/26/00.
//
// =======================================================================
 
#include "CLHEP/Random/Stat.h"
#include <cmath>
 
namespace CLHEP {
 
double HepStat::erfQ (double x) {
//
// erfQ is accurate to 7 places.
// See Numerical Recipes P 221
//
 
  double t, z, erfc; 
 
  z = std::abs(x);
  t = 1.0/(1.0+.5*z);
 
  erfc= t*std::exp(-z*z-1.26551223+t*(1.00002368+t*(0.37409196+t*(0.09678418+
    t*(-0.18628806+t*(0.27886807+t*(-1.13520398+t*(1.48851587+
    t*(-0.82215223+t*0.17087277 ))) ))) )));
 
  // (The derivation of this formula should be obvious.)
 
  if ( x < 0 ) erfc = 2.0 - erfc;
 
  return 1 - erfc;
 
}
 
 
}  // namespace CLHEP