The lower incomplete γ function: igamma

### 5.9.15 The lower incomplete γ function: igamma

The igamma command takes two mandatory arguments and an optional third argument. The mandatory arguments are a number a and a number b≥ 0. An optional third argument of 1 will return a normalized version of the function.
igamma returns the value of the incomplete γ function,

γ(a,b) = | ∫ |
b
0
| e−tta−1 dt.

With a third argument of 1, the value returned will be normalized; namely divided by Γ(a).
Input:

igamma(2.0,3.0)

Output:

0.800851726529

Input:

igamma(4.0,3.0)

Output:

2.11660866731

Input:

igamma(4.0,3.0,1)

Output:

0.352768111218

since Γ(4) = 6 and 2.11660866731/6 = 0.352768111218.