(ξ−z)∑m=0n(2m+1)Pm(z)Qm(ξ)
=
1−(ℓ+1)[Pℓ+1(z)Qℓ(ξ)−Pℓ(z)Qℓ+1(ξ)]
Abramowitz & Stegun (1995), p. 335, eq. (8.9.2)