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00001 // $Id: erfQ.cc,v 1.3 2003/08/13 20:00:12 garren Exp $ 00002 // -*- C++ -*- 00003 // 00004 // ----------------------------------------------------------------------- 00005 // HEP Random 00006 // --- erfQ --- 00007 // method implementation file 00008 // ----------------------------------------------------------------------- 00009 00010 // Contains methods that do not depend on large tables. 00011 // 00012 // erfQ (double x) 00013 00014 // ======================================================================= 00015 // M Fischler - Created 1/26/00. 00016 // 00017 // ======================================================================= 00018 00019 #include "CLHEP/Random/Stat.h" 00020 #include "CLHEP/Random/defs.h" 00021 #include <cmath> 00022 00023 namespace CLHEP { 00024 00025 double HepStat::erfQ (double x) { 00026 // 00027 // erfQ is accurate to 7 places. 00028 // See Numerical Recipes P 221 00029 // 00030 00031 double t, z, erfc; 00032 00033 z = std::abs(x); 00034 t = 1.0/(1.0+.5*z); 00035 00036 erfc= t*std::exp(-z*z-1.26551223+t*(1.00002368+t*(0.37409196+t*(0.09678418+ 00037 t*(-0.18628806+t*(0.27886807+t*(-1.13520398+t*(1.48851587+ 00038 t*(-0.82215223+t*0.17087277 ))) ))) ))); 00039 00040 // (The derivation of this formula should be obvious.) 00041 00042 if ( x < 0 ) erfc = 2.0 - erfc; 00043 00044 return 1 - erfc; 00045 00046 } 00047 00048 00049 } // namespace CLHEP 00050