# Thread: Union of open sets proof

1. ## Union of open sets proof

Problem: Prove that the union of any collection of open sets is open.

Is the following proof acceptable?

Proof: Let U' denote the collection of open sets and let U' = $\bigcup U_n$ , where n = 1,...,k. Since U' is a collection of open sets, there exists an interior point $s_0$ that belongs to U'. Since $s_0$ belongs to U', it is implied that $s_0$ belongs to at least one $U_n$. So for $s_0$ that belongs to $U_1 \in U'$, there exists a neighborhood in $U_1$; For $s_0$ that belongs to $U_2 \in U'$, there exists a neighborhood in $U_2$,...If we take the union of all $U_n$ , there exists a neighborhood in U' for all $s_0$. Therefore, U' is open.

2. ## Re: Union of open sets proof

Originally Posted by MissMousey
Problem: Prove that the union of any collection of open sets is open. Is the following proof acceptable?
Proof: Let U' denote the collection of open sets and let U' = $\bigcup U_n$ , where n = 1,...,k. Since U' is a collection of open sets, there exists an interior point $s_0$ that belongs to U'. Since $s_0$ belongs to U', it is implied that $s_0$ belongs to at least one $U_n$. So for $s_0$ that belongs to $U_1 \in U'$, there exists a neighborhood in $U_1$; For $s_0$ that belongs to $U_2 \in U'$, there exists a neighborhood in $U_2$,...If we take the union of all $U_n$ , there exists a neighborhood in U' for all $s_0$. Therefore, U' is open.
How do we know what is acceptable or not?
I am puzzled by this problem. Topologies are collections of open sets that are closed under arbitrary unions. Therefore, your notes must start at some other point. A neighborhood-space may be?
So unless you list definitions and axioms there is no way to comment.

3. ## Re: Union of open sets proof

my guess is that the thread-starter's definition of open is: a set U is open, if for every point x in U, there is a neighborhood of x that is a subset of U; or, equivalently, that every point of U is an interior point.

4. ## Re: Union of open sets proof

Originally Posted by Deveno
the thread-starter's definition of open is: a set U is open, if for every point x in U, there is a neighborhood of x that is a subset of U; or, equivalently, that every point of U is an interior point.
I understand that. The difficulty with that neighborhood-spaces are widely varied from R L Moore's developable regions to Helen Cullen's neighborhood relation.
In other words, what is a neighborhood?