(x-y)⁢∑k=0n(2⁢k+1)⁢Pk⁡(x)⁢Qk⁡(y)=(n+1)⁢(Pn+1⁡(x)⁢Qn⁡(y)-Pn⁡(x)⁢Qn+1⁡(y))-1.