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