A sequence s_n^(λ)(x) = [h(t)]^λ s_n(x), where s_n(x) is a Sheffer sequence, h(t) is invertible, and λ ranges over the real numbers is called a Steffensen sequence. If s_n(x) is an associated Sheffer sequence, then s_n^(λ) is called a cross sequence. Examples include the actuarial polynomial and Poisson-Charlier polynomial.

Appell cross sequence | Sheffer sequence | Steffensen sequence