We all know how to define the intersection and union of an arbitrary family of subsets of a set A. But what if the index set I is empty, i.e. what exactly are union and intersection of “no sets”?

[Answer: Next time. I guess most of the readers know the answer!]

## About Ken Leung

I'm a ordinary boy who likes Science and Mathematics.

I think I know the answer provided that we use a “universal set” (the largest set under consideration). But are there some subtle issues?