Fℓ⁡(η,ρ)=Cℓ⁡(η)⁢(2⁢ℓ+1)!(2⁢|η|)2⁢ℓ+1⁢ρ-ℓ⁢∑k=2⁢ℓ+1∞bk⁢tk/2⁢Jk⁡(2⁢t),