On the other hand, we've seen n that if the size of a is n, that's to say, we have n elements to choose from, then the number of possible subsets is 2 to the n.

另一方面，我们看到,如果a集合的大小是，也就是说我们有n个元素可供选择，而可能的子集的元素,个数就是2的n次方。

麻省理工公开课 - 计算机科学及编程导论课程节选