Courseware is located at Dropbox\Journey\Resources\Courses\2017-MITx-6.041x-Introduction-to-Probability.

Unit 1: Probability models and axioms

Countable additivity 一节提到：

Countable Additivity Axiom:

If A1, A2, A3,. . . is an infinite sequence of disjoint events,

then P(A1 ∪ A2 ∪ A3 ∪ · · · ) = P(A1) + P(A2) + P(A3) + · · ·

这引出了一个概念，叫 countable。sequence 意味着 countable。比如一块正方形区域中的点组成的序列，它就不是 countable 的；因为精度问题，你没法把点一个个列出来。countable 跟序列是有限还是无限的并没有关系，无限长的序列也可以是 countable。