# Riordan array

A Riordan array is an infinite lower triangular matrix, ${\displaystyle D}$, constructed out of two formal power series, ${\displaystyle d(t)}$ and ${\displaystyle h(t)}$, in such a way that ${\displaystyle d_{n,k}=[t^{n}]d(t)\left(th(t)\right)^{k}}$.