“…Let us introduce the integer s(φ, ψ, B) = max deg(ψ)−1, max(deg(φ), deg(B))− 2 . Then s = min s(φ, ψ) where the minimum is taken over all the pairs (φ, ψ, B) occurring in (2.16) is called the class of w. By extension, the integer s is also the class of {P n } n≥0 [6].…”