How can I prove that every uncountable subset of R has a limit point?

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Homework Help Overview

The original poster attempts to prove that every uncountable subset of the real numbers has a limit point. They express uncertainty about how to approach the problem, particularly in relation to the properties of the real numbers and the implications of the Bolzano-Weierstrass theorem.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the definition of a limit point and the implications of the Bolzano-Weierstrass theorem, questioning the necessity of boundedness in the context of uncountable sets. There is also a suggestion to use contradiction as a potential approach to the proof.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the problem and theorems related to limit points. Some guidance has been offered regarding the use of contradiction, but no consensus has been reached on the approach to take.

Contextual Notes

There is a noted distinction between bounded and unbounded sets, with participants questioning how this affects the existence of limit points in uncountable sets.

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Homework Statement


I am trying to prove that every uncountable subset of R has a limit point in R.

(where R is the reals)

Homework Equations


I know that the reals are dense and you can find a real in between any two reals. I feel like if you keep finding a real in between two reals, you will find a limit point. I am not even sure where to start with this idea.


The Attempt at a Solution

 
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Start with the definition of limit point. What is it?
 
Limit point of a set A: A limit point is a point x in which any neighborhood centered at x, no matter the size, intersects the set A at a point other than x.

The Bolzano-Weierstrass thrm states that , every bounded infinite subset of R has a limit point.

My problem does not have the bounded part, and is uncountable as opposed to the puny infinite.

I really don't understand why this works. Okay, if it's bounded and infinite then I feel convinced that it has a limit point (after seeing the thrm, of course). But non bounded, and uncountable? Is this problem stronger than the BW theorem?
 
Try using contradiction. Suppose your uncountable subset A in R does not have a limit point. That means, for any x in A, there is some ball around x which does not contain any element of A except itself. This should lead to a contradiction of A being uncountable.
 

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