/* gamma.c      Last Modified 10/20/98           */
/* Evaluate gamma function at integer and        */
/*   1/2 integer increments FOR SMALL ARGUMENTS! */
#include <math.h>

double gamma(double x)
{
  double y, a;
  double sqrtpi=1.7724538509055159;
 
  a=x;
  y=1.0;
  while (x>0){
    if(x>1)
      y=y*(x-1);
    else if(x==1)
      y=y;
    else if (x==0.5)
      y=y*sqrtpi;
    else{
      printf("ERROR:  gamma can only be used with integer and 1/2 int arguments\n");
      printf("ERROR:  argument was = %5.2e\n",a);
      exit(-1);
    }
    x=x-1;
  }
  return(y);
}  /* end of gamma */