A 7 Equation (A 6) can be simplified as ϕ g n − 1 = λ g n, A 8 where g n = f n − f n − 1.
A 4 Rearranging terms yields − s 2 b n n + s ( f n n − b n ( λ + ϕ ) ) + ( λ + ϕ ) f n = ϕ f n − 1 + λ f n + 1 − s ( ϕ b n − 1 + λ b n + 1 ).