The polynomials M_k(x;δ, η) which form the Sheffer sequence for g(t) | = | ({[1 + δ f(t)]^2 + [f(t)]^2})^(η/2) f(t) | = | tan(t/(1 + δ t)) which have generating function sum_(k = 0)^∞ (M_k(x;δ, η))/(k!) t^k = [(1 + δ t)^2]^(-η/2) exp((x tan^(-1) t)/(1 - δ tan^(-1) t)). The first few are

Meixner polynomial of the first kind | Sheffer sequence