$$Q\left(a,x\right)=\text{erfc}\left(\sqrt{x}\right)+\frac{e^{-x}}{\sqrt{\pi x}}\sum _{n=1}^i\frac{x^n}{\left(1-\frac{1}{2}\right)\dots \left(n-\frac{1}{2}\right)};a=i+\frac{1}{2};i\in {\mathbb{N}}^{+}$$