# Topology connectedness proof

1. Jan 4, 2010

1. The problem statement, all variables and given/known data

Let A, B be closed non-empty subsets of a topological space X with $$A \cup B$$ and $$A \cap B$$ connected.

Prove that A and B are connected.

2. Relevant equations

A set Q is not connected (disconnected) if it is expressible as a disjoint union of open sets, $$Q = S \cup T$$

3. The attempt at a solution

I'm trying a proof by contradiction.
By the above definition, a set which is not connected must be open (is this really true?). So start by assuming A is disconnected, ie $$A = C \cup D$$ for C, D open and disjoint. Then A must be open, but should also be closed. Now consider $$A \cup B = C \cup D \cup B$$ and $$A \cap B = C \cup D \cap B$$. I want to to arrive at a contradiction. The given properties are that A is both open and closed, B is closed, C and D are open and disjoint and $$A \cup B = C \cup D \cup B$$ and $$A \cap B = C \cup D \cap B$$ are connected. This all seems very complicated.

2. Jan 4, 2010

### rochfor1

No, thankfully this isn't true. The set $$[0,1] \cup [2,3]$$ is disconnected and closed. The definition of connectedness requires S and T to be relatively open; that is, open in the subspace topology induced on Q. Keeping this in mind, an equivalent definition of disconnectedness of a subspace Q is that there exist disjoint open sets S and T such that $$Q \subset S \cup T$$ and $$Q \cap S \neq \emptyset \neq Q \cap T$$. The subset relation shere removes your concern about disconnected sets being open.

Last edited: Jan 4, 2010
3. Jan 4, 2010

Ah right instead of saying "a set is not connected if..." i should have said "a topological space is not connected if...". Hmm does this mean the rest of my reasoning was wrong? I have to replace my C's and D's with intersections over X.

4. Jan 4, 2010

Ok so thanks to the push in the right direction I seem to have solved it. However I didnt use the fact that A and B or closed or that $$A \cup B$$ is connected. It was done entirely on the fact that $$A \cap B$$ is connected. Could this be right?

Last edited: Jan 4, 2010
5. Jan 4, 2010

### rochfor1

No, that's certainly not true. A counterexample is given by $$A = (1,2) \cup (2,3)$$ and $$B = [2,3)$$. Here $$A \cup B = (1, 3)$$ and $$A \cap B = (2, 3)$$ are connected, but A is not connected. Also, you should try to think of a very simple example that shows that neither A nor B must be connected even it $$A \cap B$$ is.

Last edited: Jan 4, 2010
6. Jan 5, 2010

Yeah it took me a while to realise what I missed. I showed that if A is disconnected then $$A \cap B$$ is expressible as a union of disjoint open sets, but I need to show that they are non-empty sets.