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Homework Help: Metric Space and Lindelof Theorem

  1. Oct 31, 2011 #1
    1. The problem statement, all variables and given/known data
    Assume some metric space (K,d) obeys Lindelof, take (X,d) a metric subspace of (K,d) and show it too must obey Lindelof.

    3. The attempt at a solution
    I'm assuming since I know that (K,d) obeys Lindelof then there is some open cover that has a countable subcover say {Ji | i is a positive rational number}. Since this covers all of the metric space, then it surely covers the metric subspace. So I'm thinking all I need to really prove is that the elements of this set of open countable covers is open under metric subspace (X,d)? Or do I need to show that I can select a countable number of {Ji} to cover (X,d)? If so, any suggestions on how to do so?

    This is the last assignment question, and it's due this Wednesday November 2nd, any help would be appreciated!
  2. jcsd
  3. Oct 31, 2011 #2
    It will be hard (even impossible) to prove this directly. For the simply reason that the result isn't true in an arbitrary topological space.

    You need to do a little shortcut. Do you know that in a metric space that Lindelof is equivalent to being second countable?? (prove it if you don't know)
    Well, using this, it suffices to prove that a subspace of a second countable space is second countable. This is a lot easier.
  4. Nov 1, 2011 #3
    No we haven't learned about second countable yet, so I shouldn't be using that in my proof. Any other suggestions? Thank you!
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