\[\left.\frac{\partial}{\partial\mu}\mathsf{P}^{\mu}_{\nu}\left(x\right)\right|_%
{\mu=0}=\left(\psi\left(-\nu\right)-\pi\cot\left(\nu\pi\right)\right)\mathsf{P%
}_{\nu}\left(x\right)+\mathsf{Q}_{\nu}\left(x\right),\]