第15页 set theory

we will use set theory for the mathematical model of events.outcomes of an experiment are elements of some sample space S,and each event is a subset of S.two events both occur if the outcome is in the intersection of two sets.at least one of a collection of events occurs if the outcome is the union of the sets.two events can not both occur if the sets are disjoint.an event fails to occur if the outcome is the complement of the set.the empty set stands for every event that cannot possibly occur.the collection of events is assumed to contain the sample space,the complement of each event,and the union of each countable collection of events.