(1-ⅇⅈ⁢ϕ⁢z)-λ+ⅈ⁢x⁢(1-ⅇ-ⅈ⁢ϕ⁢z)-λ-ⅈ⁢x=∑n=0∞Pn(λ)⁡(x;ϕ)⁢zn,