# Union of sets

1. Jun 6, 2013

### aaaa202

If you unite infinitely many open sets you still get an open set whilst the same is not necessarily true for a closed set. Can someone try to explain what property of a union of open sets it is, that assures that an infinite union is still open (and what property is the closed sets missing?)

2. Jun 6, 2013

### Staff: Mentor

While not directly related to your question, wikipedia discusses the clopen set:

http://en.wikipedia.org/wiki/Clopen_set

They mention that a set may be both open and closed mathematically, that the definitions of open and closed are not mutually excluse and provide examples.

3. Jun 6, 2013

### Fredrik

Staff Emeritus
In the context of topological spaces, it's a definition (of "topology" and "topological space"), so it doesn't require an explanation. In the context of metric spaces, it's easy to prove, but the details depend on what definition of "open" you're using. One very common definition says that a set is open if and only if all its elements are interior points. I suggest that you use this definition to prove it yourself. You can start the proof like this:

Let $\{E_i:i\in I\}$ be an arbitrary sequence of open sets. Let $x\in\bigcup_{i\in I}E_i$ be arbitrary.

Now you just need to show that x is an interior point of $\bigcup_{i\in I}E_i$.

For closed sets, you just need a counterexample. Consider e.g. the intervals [0+1/n,2-1/n] where n is a positive integer. What is the union of all of them?

4. Jun 6, 2013

### mathman

To some extent it depends on what you are starting with. In the abstract the open sets are defined to have the properties, closed under all unions and finite intersections. Closed sets are then defined as complements of open sets, and therefore closed under finite unions and all intersections.