\[I_{x}\left(a,b\right)\sim\tfrac{1}{2}\operatorname{erfc}\left(-\eta\sqrt{(a+b)%
/2}\right)+\frac{1}{\sqrt{2\pi(a+b)}}\*\left(\frac{x}{x_{0}}\right)^{a}\left(%
\frac{1-x}{1-x_{0}}\right)^{b}\sum_{k=0}^{\infty}\frac{(-1)^{k}c_{k}(\eta)}{(a%
+b)^{k}},\]