Let be random events in a probability space.
where represents the event “ happens for infinitely many values of .” Formally, , which is a limit superior of sets.
![$ A=[A_n \operatorname{i.o.}]$](http://images.planetmath.org:8080/cache/objects/3688/l2h/img7.png "$ A=[A_n \operatorname{i.o.}]$"){kind=small}
