$$\text{gamma\_q}\left(a,z\right)=Q\left(a,z\right)=\frac{\Gamma \left(a,z\right)}{\Gamma \left(a\right)}=\frac{1}{\Gamma \left(a\right)}\underset{z}{\overset{\infty }{\int }}{t}^{a-1}{e}^{-t}dt$$