A problem of completeness of a metric space

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Discussion Overview

The discussion revolves around a problem from Munkres' topology book concerning the completeness of a metric space. Participants explore two conditions related to the compactness of ϵ-balls and their implications for the completeness of the space.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant suggests that if every ϵ-ball in a metric space has compact closure for some fixed ϵ>0, then the space is complete.
  • Another participant notes that if for each point in the space there exists an ϵ>0 such that the corresponding ϵ-ball has compact closure, this does not guarantee completeness, and they provide a counterexample involving the sequence {1, 1/2, 1/3, ...}.
  • A third participant expresses agreement with the previous points, indicating a shared understanding of the problem's implications.

Areas of Agreement / Disagreement

Participants generally agree on the validity of the first condition leading to completeness, while there is contention regarding the second condition, with a counterexample provided to illustrate that completeness is not guaranteed.

Contextual Notes

The discussion highlights the dependence of the second condition on the choice of ϵ for each point, which may lead to different interpretations of completeness in the context of metric spaces.

Who May Find This Useful

This discussion may be useful for students and researchers in topology, particularly those interested in the properties of metric spaces and the implications of compactness and completeness.

facenian
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TL;DR
This seems to be a contradictory topology problem
Hi, I found this problem in Munkres' topology book, and it seems to be contradictory:
Let X be a metric space.
(a) Suppose that for some ϵ>0, every ϵ-Ball in X has compact closure. Show that X is complete.
(b) Suppose that for each x\in X there is an \epsilon>0 such as the ball B(x,\epsilon) has compact closure. Show by means of an example that X need not be complete.

I believe that (a) can be proved. But then, (b) must be impossible to prove. Am I crazy? or it is a typo. Any help will be much appreciated.
 
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facenian said:
Summary:: This seems to be a contradictory topology problem

(a) Suppose that for some ϵ>0, every ϵ-Ball in X has compact closure.
(b) Suppose that for each x\in X there is an \epsilon>0 such as the ball B(x,\epsilon) has compact closure.
a) ##\exists \epsilon## such that ##\forall x## ,,,

b) ##\forall x##, ##\exists \epsilon##

Note that in b), each ##\epsilon## could depend on ##x##.
 
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I think ##\{1,1/2,1/3,1/4,1/5,...\}## is a counterexample for part b.
 
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I think you are both, George and Infrared, right. Thank you very much guys.
 

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