Σ[k=1,∞]Σ[n=1,∞]|z/n|^2k
=Σ[k=1,∞]|z|^2kΣ[n=1,∞]1/n^2k
≦Σ[k=1,∞]|z|^2kΣ[n=1,∞]1/n^2
=MΣ[k=1,∞]|z|^2k (M=Σ[n=1,∞]1/n^2)
=M|z|^2/(1−|z|^2) (|z|<1)