Lower incomplete γ function

### 7.3.15 Lower incomplete γ function

The lower incomplete γ function is defined by

γ(a,b)= | ∫ |
b
0
| e−t ta−1 dt.

The igamma command computes the lower incomplete γ function.

- igamma takes two mandatory arguments and one optional argument:
  - a, a number.
  - b, a positive real number.
  - Optionally, the number 1.
- igamma(a,b) returns γ(a,b).
- igamma(a,b,1) returns a normalized version of the function; namely γ(a,b)/Γ(a).

#### Examples

igamma(4.0,3.0)

2.11660866731 |  |  |  |  |  |  |  |  |  |

igamma(4.0,3.0,1)

0.352768111218 |  |  |  |  |  |  |  |  |  |

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