We consider the family of polynomials
$$\left\{ \begin{array}{l} H_0 = 1 \\ H_k = \frac{1}{k!} \prod_{j=0}^{k-1} (X - j) \quad \text{for } k \in \llbracket 1, n \rrbracket \end{array} \right.$$
Let $k \in \mathbb{Z}$. Calculate $H_n(k)$. Distinguish three cases: $k \in \llbracket 0, n-1 \rrbracket$, $k \geqslant n$, and $k < 0$. For the latter case, set $k = -p$.