double
cephes_igamc(double a, double x)
{
double ans, ax, c, yc, r, t, y, z;
double pk, pkm1, pkm2, qk, qkm1, qkm2;
if ( (x <= 0) || ( a <= 0) )
return( 1.0 );
if ( (x < 1.0) || (x < a) )
return( 1.e0 - cephes_igam(a,x) );
ax = a * log(x) - x - cephes_lgam(a);
if ( ax < -MAXLOG ) {
printf("igamc: UNDERFLOW\n");
return 0.0;
}
ax = exp(ax);
/* continued fraction */
y = 1.0 - a;
z = x + y + 1.0;
c = 0.0;
pkm2 = 1.0;
qkm2 = x;
pkm1 = x + 1.0;
qkm1 = z * x;
ans = pkm1/qkm1;
do {
c += 1.0;
y += 1.0;
z += 2.0;
yc = y * c;
pk = pkm1 * z - pkm2 * yc;
qk = qkm1 * z - qkm2 * yc;
if ( qk != 0 ) {
r = pk/qk;
t = fabs( (ans - r)/r );
ans = r;
}
else
t = 1.0;
pkm2 = pkm1;
pkm1 = pk;
qkm2 = qkm1;
qkm1 = qk;
if ( fabs(pk) > big ) {
pkm2 *= biginv;
pkm1 *= biginv;
qkm2 *= biginv;
qkm1 *= biginv;
}
} while ( t > MACHEP );
return ans*ax;
}
static double
gammainc(double x, double params[2])
{
return cephes_igam(params[0], x) - params[1];
}
