Find in Library

In this paper, we use elementary method and some sort of a counting argument to show the equality of two expressions. That is, let $ f(n) $ and $ g(n) $ be two functions, $ k $ be any positive integer. Then $ f(n) = \sum\limits_{r = 0}^n(-1)^r\cdot \frac{n!}{r!}\cdot \binom{n+k-1}{r+k-1}\cdot g(r) $ if and only if $ g(n) = \sum\limits_{r = 0}^n(-1)^r\cdot \frac{n!}{r!}\cdot \binom{n+k-1}{r+k-1}\cdot f(r) $ for all integers $ n\geq0 $. As an application of this formula, we obtain some new identities involving the famous Laguerre polynomials.