# Thread: Intro to topology and open sets

1. ## Intro to topology and open sets

Hello All,

Topology is just getting under way for me, and I have a question.

Consider a set {n}. Is it open, closed, or neither? Specifically, say {8}, for example.

I say this set is closed, because it has upper and lower bounds at {8}. Also, as I understand it, the compliment {(-inf,8) u (8, inf)} is open. Thoughts?

All input is greatly appreciated.

SY

2. ## Re: Intro to topology and open sets

Originally Posted by SYoungblood
Consider a set {n}. Is it open, closed, or neither? Specifically, say {8}, for example.
I say this set is closed,
because it has upper and lower bounds at {8}. Also, as I understand it, the compliment {(-inf,8) u (8, inf)} is open. Thoughts?
With knowing the exact set of axioms in use, it is difficult to answer.
However, in the usual topology on the set of real numbers any finite set is closed.
The complement of a finite set is open so the set is closed.

3. ## Re: Intro to topology and open sets

Thank you kindly, my text didn't define discrete topology until the section after where I came across this little idea. I think I am good to go now. Relatively speaking.

4. ## Re: Intro to topology and open sets

Well, did your text define "topology"? It's impossible to define "open" or "closed" sets without having a topology and, for this question, it is necessary to say, explicitly, which topology, on this set, you are using.

5. ## Re: Intro to topology and open sets

Additionally, if you have $A\subseteq B$, then the subset is typically assumed to take the topology of the superset. However, if your full set (sometimes called a universe) is some set $A$, then regardless of the contents of $A$, if it has a topology, it is a clopen subset of itself.

Example, $A = [1,2] \cup (3,4] \cup [5,6) \cup (7,8) \cup \{9\}$ is neither an open nor a closed subset of the reals, but it is a clopen subset of itself in any topology (typically, the inherited topology from the reals).