Here's the problem. How do I get to the nth- er, the k'th element in the list, in this case?

如何找到第n个元素呢-,在这里，如何找到第k个元素呢？

麻省理工公开课 - 计算机科学及编程导论课程节选

So sometimes what we do is introduce a little vector here called er.

通常我们的做法就是引入一个矢量 er

耶鲁公开课 - 基础物理课程节选