Topology, counter examples needed.

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Homework Help Overview

The discussion revolves around finding counterexamples in topology to demonstrate the failure of two specific theorems regarding compactness and Hausdorff spaces. The original poster seeks examples of a non-compact space where every closed subset is compact, and a non-Hausdorff space where every compact subset is closed.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to identify non-compact spaces and non-Hausdorff spaces that meet the criteria for the counterexamples but expresses difficulty in finding suitable examples.
  • Some participants question the validity of the original poster's assumptions regarding the theorems and their opposites.
  • One participant suggests the cocountable topology as a potential example for the second theorem.

Discussion Status

The discussion is ongoing, with participants exploring the validity of the original poster's claims and the potential for counterexamples. There is a recognition of the complexity of the task, and some guidance has been offered regarding possible examples.

Contextual Notes

The original poster mentions that they have proven the opposite theorems, which hold true, and are tasked with demonstrating why the original claims do not hold. This context suggests a structured approach to the problem, but specific constraints or assumptions are not fully detailed.

Tomer
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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.
 
Last edited:
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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.
 
Last edited by a moderator:
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 :-)
 
For (2), you might want to try the cocountable topology.
 
Thanks, I will! :-)
 

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