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cdf_36.c

00001 #include "cdflib.h"
00002 double dbetrm(double *a,double *b)
00003 /*
00004 **********************************************************************
00005  
00006      double dbetrm(double *a,double *b)
00007           Double Precision Sterling Remainder for Complete
00008                     Beta Function
00009  
00010  
00011                               Function
00012  
00013  
00014      Log(Beta(A,B)) = Lgamma(A) + Lgamma(B) - Lgamma(A+B)
00015      where Lgamma is the log of the (complete) gamma function
00016  
00017      Let ZZ be approximation obtained if each log gamma is approximated
00018      by Sterling's formula, i.e.,
00019      Sterling(Z) = LOG( SQRT( 2*PI ) ) + ( Z-0.5 ) * LOG( Z ) - Z
00020  
00021      Returns Log(Beta(A,B)) - ZZ
00022  
00023  
00024                               Arguments
00025  
00026  
00027      A --> One argument of the Beta
00028                     DOUBLE PRECISION A
00029  
00030      B --> The other argument of the Beta
00031                     DOUBLE PRECISION B
00032  
00033 **********************************************************************
00034 */
00035 {
00036 static double dbetrm,T1,T2,T3;
00037 /*
00038      ..
00039      .. Executable Statements ..
00040 */
00041 /*
00042      Try to sum from smallest to largest
00043 */
00044     T1 = *a+*b;
00045     dbetrm = -dstrem(&T1);
00046     T2 = fifdmax1(*a,*b);
00047     dbetrm += dstrem(&T2);
00048     T3 = fifdmin1(*a,*b);
00049     dbetrm += dstrem(&T3);
00050     return dbetrm;
00051 } /* END */

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cdf_36.c