\[(1-{\mathrm{e}}^{\mathrm{i}\phi}z)^{-\lambda+\mathrm{i}x}(1-{\mathrm{e}}^{-%
\mathrm{i}\phi}z)^{-\lambda-\mathrm{i}x}=\sum_{n=0}^{\infty}P^{(\lambda)}_{n}%
\left(x;\phi\right)z^{n},\]