κn⁢ϕn⁡(0)⁢ϕn+1⁡(z)+κn-1⁢ϕn+1⁡(0)⁢z⁢ϕn-1⁡(z)=(κn⁢ϕn+1⁡(0)+κn+1⁢ϕn⁡(0)⁢z)⁢ϕn⁡(z).