Chandra X-Ray Observatory
Skip to the navigation links
Last modified: December 2010

Jump to: Description · Examples · Bugs · See Also

AHELP for CIAO 4.3 Sherpa v1


Context: utilities


Calculate the compliment of the incomplete Gamma function (upper)




Calculate the CEPHES function, igamc, in the range [a > 0; x > 0]. The function is defined as

The function, igamc, is defined by

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

           = 1/sqrt(a) Int_(x)^(Inf) e^(-t) t^(a-1) dt

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. Arguments can be in scalar or array form.


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


Cephes Math Library Release 2.0: April, 1987. Copyright 1985, 1987 by Stephen L. Moshier. Direct inquiries to 30 Frost Street, Cambridge, MA 02140.

Example 1

sherpa> igamc(1,2)

Calculate igamc with a=1, x=2.

Example 2

sherpa> igamc([1,2],[2,3])

Calculate igamc with a=[1,2], x=[2,3] .


See the bugs pages on the Sherpa website for an up-to-date listing of known bugs.

See Also

calc_chisqr, calc_energy_flux, calc_model_sum, calc_photon_flux, calc_source_sum, calc_stat, gamma, igam, incbet, lgam

Last modified: December 2010
Smithsonian Institute Smithsonian Institute

The Chandra X-Ray Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory. 60 Garden Street, Cambridge, MA 02138 USA.   Email: Smithsonian Institution, Copyright © 1998-2019. All rights reserved.