Let $\left( u _ { n } \right) _ { n \in \mathbb { N } } \in \mathbb { R } ^ { \mathbb { N } }$ and $\alpha \in \mathbb { R } ^ { * }$. Suppose that $\lim _ { n \rightarrow + \infty } e _ { n } = \alpha$, where $e_n = u_{n+1} - u_n$. Using (Cesàro), give an equivalent of $\left( u _ { n } \right) _ { n \in \mathbb { N } }$. Recover this result using a comparison theorem for series with positive terms.