And, of course, as with functions of one variable, we don't need things to be defined everywhere.

当然，就像单变量的函数一样，我们不需要函数在任意处都有定义。

youdao

Sometime later, I will deal with functions of more than one variable, which I will briefly introduce to you, because that may not be a prerequisite but certainly something you will learn and you may use on and off.

再过段时间,我要开始处理多变量函数,到时候我会向你们进行简单介绍,因为那不是必须的,但是你们以后会学到,有时候还能派上用场

耶鲁公开课 - 基础物理课程节选