\[U\left(a,b,b\right)=\sqrt{\pi}\left(2b\right)^{-\frac{1}{2}a}\left(\frac{1}{%
\Gamma\left(\frac{1}{2}(a+1)\right)}-\frac{(a+1)\sqrt{8/b}}{3\Gamma\left(\frac%
{1}{2}a\right)}+O\left(\frac{1}{b}\right)\right).\]