\[U\left(a,b,z\right)\sim 2\left(z/a\right)^{(1-b)/2}\frac{e^{z/2}}{\Gamma\left(%
a\right)}\*\left(K_{b-1}\left(2\sqrt{az}\right)\sum_{s=0}^{\infty}\frac{p_{s}(%
z)}{a^{s}}+\sqrt{z/a}K_{b}\left(2\sqrt{az}\right)\sum_{s=0}^{\infty}\frac{q_{s%
}(z)}{a^{s}}\right),\]