igam

Synopsis

Calculate the regularized incomplete Gamma function (lower)

igam(a,x)

Description

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

igam(a,x) = 1/gamma(a) Int_(0)^(x) e^(-t) t^(a-1) dt

where "gamma(a)" represents the complete Gamma function.

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.

Relative error:

Reference

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> igam(1,2)

Calculate the igam of a=1, x=2 .

Example 2

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

Calculate the igam of a=[1,2], x=[2,3] .

Bugs

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

See Also

tools

aprates, modelflux

utilities

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