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