## lim sup & lim inf of Sets

The concept of lim sup and lim inf can be applied to sets too. Here is a nice characterisation of lim sup and lim inf of sets:

For a sequence of sets $\{E_k\}$, $\limsup E_k$ consists of those points that belong to infinitely many $E_k$, and $\liminf E_k$ consists of those points that belong to all $E_k$ from some $k$ on (i.e. belong to all but finitely many $E_k$).

Proof:
Note that
\begin{aligned} x\in\limsup E_k&\iff x\in\bigcup_{k=j}^\infty E_k\ \text{for all}\ j\in\mathbb{N}\\ &\iff\text{For all}\ j\in\mathbb{N}, \text{there exists}\ i\geq j\ \text{such that}\ x\in E_i\\ &\iff x\ \text{belongs to infinitely many}\ E_k. \end{aligned}
\begin{aligned} x\in\liminf E_k&\iff x\in\bigcap_{k=j}^\infty E_k\ \text{for some}\ j\in\mathbb{N}\\ &\iff x\in E_k\ \text{for all}\ k\geq j. \end{aligned}