Prove that the only sets that are both open and closed are and the empty set .

How do you go about proving that every other set is either open, closed, or neither? There are infinitely many other sets.

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- October 18th 2010, 11:29 AMZalrenBoth open and closed set proof
Prove that the only sets that are both open and closed are and the empty set .

How do you go about proving that every other set is either open, closed, or neither? There are infinitely many other sets. - October 18th 2010, 11:49 AMPlato
What do you know about connected sets? The space is connected.

If , is non-trivial, and is both open and closed then .

Is that a contradiction? WHY? - October 18th 2010, 12:10 PMZalren
If is open then is closed. So if a set is both open and closed, what does that make the complement of that set? Neither open nor closed?

- October 18th 2010, 12:17 PMPlato
No. These are known as

*clopen*sets (both open and closed).

So if is clopen then is clopen.

There is a well known result about connected sets and clopen sets.

Otherwise, I have not seen this topic discussed. - October 18th 2010, 12:29 PMZalren
So if I can prove that the empty set, , is both open and closed (or clopen) then I can say that the set of all reals, , is clopen because is the complement to ?

- October 18th 2010, 12:57 PMPlato
It is completely obvious that those two sets are both open and closed.

They are the only two sets in that are.