Give an example, or argue that it is impossible

  • Thread starter Thread starter davitykale
  • Start date Start date
  • Tags Tags
    Example Impossible
Click For Summary

Homework Help Overview

The discussion revolves around the properties of open sets in the real numbers, specifically exploring the conditions under which the closure of an open set A equals R while the complement R \ A remains uncountable.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the implications of a set being dense in R and question the validity of the original statement. There are attempts to connect the concept to the Cantor set and its properties, with some participants expressing uncertainty about the openness of the Cantor set.

Discussion Status

The conversation is active, with participants exploring different interpretations of the problem. Some guidance has been offered regarding the relationship between the Cantor set and the example being sought, but no consensus has been reached on the original claim.

Contextual Notes

There is a mention of the Cantor set and its complement, as well as the need to clarify the definitions of open sets and their closures in the context of the problem.

davitykale
Messages
38
Reaction score
0

Homework Statement


An open set A contained in R (reals) such that the closure of A = R, but R \ A is uncountable


Homework Equations


I guess knowing that for the closure of a set A to be equal to R means that A is dense in R?


The Attempt at a Solution


Every thing I try seems to fail, but I have absolutely no idea how I would go about even trying to prove that this wasn't possible
 
Physics news on Phys.org
I think it's false and I want to try and argue this. Can anyone help me get started on a proof?
 
It's not false. Do you know the Cantor set?
 
Oh, I do know the Cantor's set! That makes a lot of sense. R - Cantor's set is uncountable because they are both uncountable?
 
Wait...the Cantor set is open?
 
davitykale said:
Wait...the Cantor set is open?

Not at all. What set related to the Cantor set is open?
 
It's complement? The union of the open intervals that are removed?
 
davitykale said:
It's complement? The union of the open intervals that are removed?

Right. R-Cantor set is the sort of example you are looking for.
 
That makes a lot of sense. Thanks!
 

Similar threads

  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 29 ·
Replies
29
Views
4K
Replies
2
Views
2K
Replies
1
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K