Topology, counter examples needed.

In summary, Tomer was trying to find a counter example to the theorem that if a space is compact, then every closed subset is compact. He couldn't find one, and then he was told that the opposite might be true.
  • #1
Tomer
202
0

Homework Statement



I need two counter examples, that show the following two theorems don't/B] hold:
Let X be a topological space.

1. If from the closeness of any subset A in X follows compactness of A, then X is compact.
2. If from the compactness of a subset A in X follows closeness of A, then X is housdorff.

I proved the *opposite* theormes which do hold, but I cannot seem to find counter examples.

That means, I need to find a non-compact space, in which every closed subset is compact, and a non-housdorff space, in which every compact subset in closed.

Homework Equations



Are there any equations in Topology?

The Attempt at a Solution



I just tried to take for "1" spaces I know that are not compact, but then couldn't find a space in which every closed subset is a compact one...
In "2" I thought of non-hausdorff spaces I know, but couldn't directly see whether indeed every compact set is closed.

I'd really appreciate your help!

Thanks,
Tomer.
 
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  • #2
You aren't going to be able to find a counter example to (1) because it is true. Every space is closed in itself. If it is true that every closed subset of A is compact, then A itself is compact because A is a closed subset of itself.
 
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  • #3
Thanks a lot for the reply.
I see you're point :-) That is strange - in the task we were to prove the next theorem (part a):
If X is compact, it follows from the closeness of A that A is compact.

Then we need to give counter examples to show why the opposite doesn't hold. I translated the "opposite claim" correctly, right?

But I definitely agree with what you just said :-)
 
  • #4
For (2), you might want to try the cocountable topology.
 
  • #5
Thanks, I will! :-)
 

FAQ: Topology, counter examples needed.

1. What is topology?

Topology is a branch of mathematics that studies the properties of geometric objects that are preserved under continuous deformations.

2. What are some applications of topology?

Topology has various applications in different fields such as physics, biology, computer science, and economics. It is used to study the shape of molecules, the structure of materials, and the behavior of data networks, among other things.

3. What is a counter example in topology?

A counter example in topology is a specific example that disproves a certain conjecture or statement about topological spaces. It shows that the statement is not true in all cases.

4. Why is the use of counter examples important in topology?

Counter examples help to identify the limitations of certain theorems and definitions in topology. They also aid in understanding the properties and characteristics of different topological spaces.

5. Can you give an example of a counter example in topology?

One example of a counter example in topology is the Long Line, which is a topological space that is not compact, even though it satisfies the other properties of a compact space. This contradicts the statement that all spaces satisfying those properties are compact.

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