\[c(\kappa,\mu)=e^{\mu\pi\mathrm{i}}\sqrt{\tfrac{1}{2}\pi}\left(\frac{\kappa-\mu%
}{\kappa+\mu}\right)^{\frac{1}{2}\mu}\left(\frac{e^{2}}{\kappa^{2}-\mu^{2}}%
\right)^{\frac{1}{2}\kappa},\]