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