In mathematics, an Appell sequence, named after Paul Émile Appell, is any polynomial sequence { p n } n = 0, 1, 2, … {\displaystyle \{p_{n}\}_{n=0,1,2,\ldots }} satisfying the identity d d x p n = n p n − 1, {\displaystyle {\frac {d}{dx}}p_{n}=np_{n-1},} and in which p 0 {\displaystyle p_{0}} is a non-zero constant. Wikipedia