Prove the characterization of lim sup given above. That is, given a bounded sequence $\left(a_n\right)$, show that the number $M=\lim \sup _{n \rightarrow \infty} a_n$ satisfies ( $*$ ) and, conversely, that any number $M$ satisfying (*) must equal $\lim \sup _{n \rightarrow \infty} a_n$. State and prove the corresponding result for $m=\liminf _{n \rightarrow \infty} a_n$.