GammaContinuedFraction.h File Reference

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Functions
float	GammaContinuedFraction (float a, float x)

Function Documentation

GammaContinuedFraction()

float GammaContinuedFraction	(	float	a,
		float	x
	)

Returns the continued fraction summation of the (complement of the) incomplete gamma function P(a,x) evaluated by its continued fraction representation. source: Numerical Recipes

Definition at line 9 of file GammaContinuedFraction.cc.

References a, b, HltBtagPostValidation_cff::c, DMR_cfg::cerr, ztail::d, EPS, FPMIN, h, mps_fire::i, ITMAX, and x.

Referenced by IncompleteGammaComplement::ln(), and IncompleteGammaComplement::value().

                                               {
  int i;
  float an, del;

  /* Set up for evaluating continued fraction by modified Lentz's method (par.5.2
     in Numerical Recipes in C) with b_0 = 0 */
  double b = x + 1.0 - a;
  double c = 1.0 / FPMIN;
  double d = 1.0 / b;
  double h = d;
  for (i = 1; i <= ITMAX; i++) {
    an = -i * (i - a);
    b += 2.0;
    d = an * d + b;
    if (fabs(d) < FPMIN)
      d = FPMIN;
    c = b + an / c;
    if (fabs(c) < FPMIN)
      c = FPMIN;
    d = 1.0 / d;
    del = d * c;
    h *= del;
    if (fabs(del - 1.0) < EPS)
      break;
  }
  if (i > ITMAX)
    std::cerr << "GammaContinuedFraction::a too large, "
              << "ITMAX too small" << std::endl;
  return h;
}