For an application $f : \mathbb{R}_+^* \rightarrow \mathbb{R}$ of class $\mathcal{C}^\infty$, we define the application
$$\delta(f) : \left\{ \begin{array}{l} \mathbb{R}_+^* \rightarrow \mathbb{R} \\ x \mapsto f(x+1) - f(x) \end{array} \right.$$
Show that $\delta(f)$ is of class $\mathcal{C}^\infty$ on $\mathbb{R}_+^*$. Compare $(\delta(f))'$ and $\delta(f')$.