Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Totally bounded sets homework

  1. Mar 29, 2010 #1

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Not really homework, but a textbook-style question...

    1. The problem statement, all variables and given/known data
    Is every subset of a totally bounded set (of a metric space) totally bounded?


    2. Relevant 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.

    3. 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.
     
  2. jcsd
  3. Mar 29, 2010 #2

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    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?
     
  4. Mar 29, 2010 #3

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook