Undergrad A problem of completeness of a metric space

Click For Summary
The discussion centers on a problem from Munkres' topology book regarding the completeness of a metric space. Part (a) asserts that if every ϵ-ball in a metric space X has compact closure for some fixed ϵ>0, then X is complete. Part (b) states that if for each point x in X there exists an ϵ>0 such that the ball B(x,ϵ) has compact closure, X may not be complete, with a counterexample provided. The participants express confusion over the apparent contradiction between the two parts, with some suggesting that part (b) is indeed valid despite the potential for misunderstanding. The conversation highlights the nuances in the definitions of compactness and completeness in metric spaces.
facenian
Messages
433
Reaction score
25
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.
 
Physics news on Phys.org
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##.
 
  • Like
Likes PeroK and facenian
I think ##\{1,1/2,1/3,1/4,1/5,...\}## is a counterexample for part b.
 
  • Like
Likes PeroK and facenian
I think you are both, George and Infrared, right. Thank you very much guys.
 

Similar threads

  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 16 ·
Replies
16
Views
2K
  • · Replies 17 ·
Replies
17
Views
1K
  • · Replies 32 ·
2
Replies
32
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 0 ·
Replies
0
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 7 ·
Replies
7
Views
3K