igamc

Complemented incomplete gamma integral.

SYNOPSIS:

double a, x, y, igamc();
y = igamc(a, x);

DESCRIPTION:

The function is defined by

igamc(a,x)   =   1 - igam(a,x)

                            inf.
                              -
                     1       | | -t a-1
               =   -----     |   e   t    dt
                    -      | |
                   | (a)    -
                             x

In this implementation both arguments must be positive. The integral is evaluated by either a power series or continued fraction expansion, depending on the relative values of a and x.

ACCURACY:

Tested at random a, x.
                a         x                      Relative error:
arithmetic   domain   domain     # trials      peak         rms
    IEEE     0.5,100   0,100      200000       1.9e-14     1.7e-15
    IEEE     0.01,0.5 0,100      200000       1.4e-13     1.6e-15