\[W\left(a,-x\right)=\frac{-i}{\sqrt{2k}}\,e^{\frac{1}{4}\pi a}\left(e^{i\rho}U%
\left(ia,xe^{-\pi i/4}\right)-e^{-i\rho}U\left(-ia,xe^{\pi i/4}\right)\right),\]