i/gamma

Home Manual Packages Global Index Keywords Quick Reference |
    /*
       GAMMA.I

       Gamma function, beta function, and binomial coefficients.

       $Id$
     */
    /*    Copyright (c) 1994.  The Regents of the University of California.
                        All rights reserved.  */

    /* Algorithms from Numerical Recipes, Press, et. al., section 6.1,
       Cambridge University Press (1988).
     */

    func ln_gamma (z)
    /* DOCUMENT ln_gamma(z)

         returns natural log of the gamma function.
         Error is less than 2.e-10 for real part of z>=1.

         Use lngamma if you know that all z>=1, or if you don't care much about
         accuracy for z<1.

       SEE ALSO: lngamma, bico, beta
     */
    {

      if (structof(z)==complex) big= (z.re>=1.0);
      else big= (z>=1.0);
      list= where(big);

      if (numberof(list)) lg1= lngamma(z(list));
      list= where(!big);
      if (numberof(list)) {
        z= z(list);

        lg2= log(pi/sin(pi*z)) - lngamma(1.0-z)
      }
      return merge(lg1,lg2,big);
    }

    func lngamma (x)
    /* DOCUMENT lngamma(x)

         returns natural log of the gamma function.
         Error is less than 2.e-10 for real part of x>=1.
         Use ln_gamma if some x<1.

       SEE ALSO: ln_gamma, bico, beta
     */
    {
      tmp= x+4.5;
      tmp-= (x-0.5)*log(tmp);
      ser= 1.0 + 76.18009173/x - 86.50532033/(x+1.) + 24.01409822/(x+2.) -
        1.231739516/(x+3.) + 0.120858003e-2/(x+4.) - 0.536382e-5/(x+5.);
      return -tmp+log(2.50662827465*ser);
    }

    func bico (n,k)
    /* DOCUMENT bico(n,k)
         returns the binomial coefficient n!/(k!(n-k)!) as a double.

       SEE ALSO: ln_gamma, beta
     */
    {

      return floor(0.5+exp(lngamma(n+1.)-lngamma(k+1.)-lngamma(n-k+1.)));
    }

    func beta (z,w)
    /* DOCUMENT beta(z,w)

         returns the beta function gamma(z)gamma(w)/gamma(z+w)

       SEE ALSO: ln_gamma, bico
     */
    {

      return exp(lngamma(z)+lngamma(w)-lngamma(z+w));
    }

    func erfc(x)
    /* DOCUMENT erfc(x)

         returns the complementary error function 1-erf with fractional

         error less than 1.2e-7 everywhere.
     */
    {
      z= abs(x);
      t= 1.0/(1.0+0.5*z);
      ans= t*exp(-z*z +

    	     poly(t, -1.26551223, 1.00002368, 0.37409196, 0.09678418,
    		  -0.18628806, 0.27886807, -1.13520398, 1.48851587,
    		  -0.82215223, 0.17087277));
      z= sign(x);
      return ans*z + (1.-z);
    }