Excluded Point Topology: Int(A) and Cl(A) for Sets A with or without p in X

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Homework Help Overview

The discussion revolves around the excluded point topology on a set X, specifically focusing on determining the interior (Int(A)) and closure (Cl(A)) of sets A that either contain or do not contain a specific point p in X.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the definitions of interior and closure in the context of the excluded point topology. There are attempts to clarify whether the interior of A would be A minus p when p is included in A, and whether A itself would be considered open or closed. Questions are raised about the smallest closed set containing A and the implications of p's presence or absence in A.

Discussion Status

The discussion is ongoing, with participants sharing their thoughts on the properties of closure and interior. Some guidance has been offered regarding the definitions of these concepts, but no consensus has been reached on the specific cases being analyzed.

Contextual Notes

Participants are working under the constraints of the excluded point topology, which defines open sets as those that exclude the point p. There is also a focus on the need for clarity in organizing responses and the potential use of proof techniques such as proof by contradiction.

cragar
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Homework Statement


Consider the excluded point topology on a set X.
Determine Int(A) and Cl(A) for sets A containing p and for sets A not containing p.
Excluded point topology is all the subsets of X that exclude p.
where p is in X.

The Attempt at a Solution


So the interior of a set A is the union of all open sets contained in A.
Would the interior for A be A-p , where we exclude p.
and would the interior be A where we include p.
I am not sure what the smallest closed set would be that contains A.
It seem like it would just be A.
 
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cragar said:

Homework Statement


Consider the excluded point topology on a set X.
Determine Int(A) and Cl(A) for sets A containing p and for sets A not containing p.
Excluded point topology is all the subsets of X that exclude p.
where p is in X.

The Attempt at a Solution


So the interior of a set A is the union of all open sets contained in A.
Would the interior for A be A-p , where we exclude p.
and would the interior be A where we include p.
I'm not sure what the smallest closed set would be that contains A.
It seem like it would just be A.

It would be helpful to organize you answers better.

If p∈A,
Determine Int(A).

Determine Cl(A).​

If p∉A,
Determine Int(A).

Determine Cl(A).​
 
for the closure of those sets, should I try a proof by contradiction.
for the second one assume that p is not in the closure.Since p is not in A it is in the complement so it is in a closed set.
 
Have you proven any properties for closure and interior? For example, you can show that interior of A is the largest open set contained in A and closure of A is the smallest closed set which contains A. If you can prove this, then you're almost done.
 
cragar said:
for the closure of those sets, should I try a proof by contradiction.
for the second one assume that p is not in the closure.Since p is not in A it is in the complement so it is in a closed set.
Start with some basics.

If p\in\text{A}\,, then is set A open or is A closed?

If p\notin\text{A}\,, then is set A open or is A closed?
 

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