The power polynomials x^n are an associated Sheffer sequence with f(t) = t, giving generating function sum_(k = 0)^∞ x^k t^k = 1/(1 - t x) and exponential generating function sum_(k = 0)^∞ x^k/(k!) t^k = e^(x t) and binomial identity (x + y)^n = sum_(k = 0)^n(n k) x^k y^(n - k).