\[\sqrt{2\pi}U\left(a,z\right)=\Gamma\left(\tfrac{1}{2}-a\right)\left(e^{\mp i%
\pi(\frac{1}{2}a+\frac{1}{4})}U\left(-a,\pm iz\right)+e^{\pm i\pi(\frac{1}{2}a%
+\frac{1}{4})}U\left(-a,\mp iz\right)\right),\]