Output of file : IGAMI.C contained in archive : CEPHES22.ZIP
/* igami()
*
* Inverse of complemented imcomplete gamma integral
*
*
*
* SYNOPSIS:
*
* double a, x, y, igami();
*
* x = igami( a, y );
*
*
*
* DESCRIPTION:
*
* Given y, the function finds x such that
*
* igamc( a, x ) = y.
*
* Starting with the approximate value
*
* 3
* x = a t
*
* where
*
* t = 1 - d - ndtri(y) sqrt(d)
*
* and
*
* d = 1/9a,
*
* the routine performs up to 10 Newton iterations to find the
* root of igamc(a,x) - y = 0.
*
*
* ACCURACY:
*
* Tested for a ranging from 0.5 to 30 and x from 0 to 0.5.
*
* Relative error:
* arithmetic domain # trials peak rms
* DEC 0,0.5 3400 8.8e-16 1.3e-16
* IEEE 0,0.5 10000 1.1e-14 1.0e-15
*
*/

/*
Cephes Math Library Release 2.0: April, 1987
Copyright 1984, 1987 by Stephen L. Moshier
Direct inquiries to 30 Frost Street, Cambridge, MA 02140
*/

#include "mconf.h"

extern double MACHEP, MAXNUM, MAXLOG, MINLOG;

double igami( a, y0 )
double a, y0;
{
double d, y, x0, lgm;
int i;
double igamc();
double ndtri(), exp(), fabs(), log(), sqrt(), lgam();

/* approximation to inverse function */
d = 1.0/(9.0*a);
y = ( 1.0 - d - ndtri(y0) * sqrt(d) );
x0 = a * y * y * y;

lgm = lgam(a);

for( i=0; i<10; i++ )
{
if( x0 <= 0.0 )
{
mtherr( "igami", UNDERFLOW );
return(0.0);
}
y = igamc(a,x0);
/* compute the derivative of the function at this point */
d = (a - 1.0) * log(x0) - x0 - lgm;
if( d < -MAXLOG )
{
mtherr( "igami", UNDERFLOW );
goto done;
}
d = -exp(d);
/* compute the step to the next approximation of x */
if( d == 0.0 )
goto done;
d = (y - y0)/d;
x0 = x0 - d;
if( i < 3 )
continue;
if( fabs(d/x0) < 2.0 * MACHEP )
goto done;
}

done:
return( x0 );
}

### 3 Responses to “Category : C Source CodeArchive   : CEPHES22.ZIPFilename : IGAMI.C”

1. Very nice! Thank you for this wonderful archive. I wonder why I found it only now. Long live the BBS file archives!

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