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Totally bounded sets homework

  • Thread starter Fredrik
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  • #1
Fredrik
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Not really homework, but a textbook-style question...

Homework Statement


Is every subset of a totally bounded set (of a metric space) totally bounded?


Homework Equations



F is said to be totally bounded if, for every [itex]\epsilon>0[/itex], there's a finite subset [itex]F_0\subset F[/itex] such that [tex]F\subset\bigcup_{x\in F_0}B(x,\epsilon)[/tex], where [itex]B(x,\epsilon)[/itex] is the open ball of radius [itex]\epsilon[/itex] around x.

The Attempt at a Solution


Suppose that [itex]E\subset F[/itex], and that F is totally bounded. Let [itex]\epsilon>0[/itex] be arbitrary. We know that there exists a finite set [itex]F_0\subset F[/itex] such that [tex]E\subset F\subset\bigcup_{x\in F_0}B(x,\epsilon)[/tex], but this doesn't seem to help, since [itex]F_0[/itex] doesn't have to be a subset of E. We might even have [itex]F_0\cap E=\emptyset[/itex]. So now I'm starting to think that maybe E doesn't have to be totally bounded at all. For example, if F is some open ball in [itex]\mathbb R^2[/itex] and E is some kind of fractal or something.
 

Answers and Replies

  • #2
Dick
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No, a totally bounded subset of a totally bounded set is totally bounded. Most definitions are a bit looser than yours. But you can still prove it. Pick a finite set F0 to cover with balls of radius e/2. Can you use that to construct a finite set E0 that covers E with radius e?
 
  • #3
Fredrik
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Ah, I get it now. I'll just pick one point from each non-empty [itex]B(x,\epsilon/2)\cap E[/itex] with [itex]x\in F_0[/itex], and take those points to be my E0. Then I consider open balls around those points, and I need to take these balls to have twice the radius to ensure that they cover E (by covering the old balls that had non-empty intersection with E). Thanks.
 

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