gammp
gammp (a, x [, accuracy ])
The gammp function returns the regularized incomplete gamma function P(a,x), where a > 0, x >= 0. Optionally, accuracy can be used to specify the desired fractional accuracy. Same as gammaInc(a, x, 0)/gamma(a).
Details
The accuracy parameter specifies the fractional accuracy that you desire. That is, if you set accuracy to 1e-7, that means that you wish that the absolute value of (factual-freturned)/factual be less than 10-7.
For backward compatibility, if you don't include accuracy , gammp uses older code that is slower for an equivalent accuracy, and cannot achieve as high accuracy.
The ability of gammp to return a value having full fractional accuracy is limited by double-precision calculations. This means that it will mostly have fractional accuracy better than about 10-15, but this is not guaranteed, especially for extreme values of a and x.