Metrizable space with countable dense subset

  1. radou

    radou 3,217
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    1. The problem statement, all variables and given/known data

    Show that every metrizable space with a countable dense subset has a countable basis.

    3. The attempt at a solution

    Let A be a countable dense subset of the metric space (X, d). For any x in A, take the countable collection of open balls {B(x, 1/n) : n is a positive integer}. Since A is countable, the family (call it F) of all such open balls is countable. We claim that it is a basis for X.

    First of all, F covers X. Since Cl(A) = X, for any x0 in X, there exists a sequence xn of points of A such that xn --> x. So, for any ε > 0, there exists a positive integer N such that for n >= N, xn is in the open ball B(x0, ε). But now, for any such xn, the open ball B(xn, ε) contains x0, so F indeed covers all of X.

    Clearly if x is in the intersection of any two elements of F, there exists another element of F (i.e. some positive integer N) such that B(x, 1/N) is contained in the intersection.
     
  2. jcsd
  3. micromass

    micromass 18,702
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    All is good. But your not done. You still need to show that your basis generates the correct topology. All you've proven now is that it's a basis. But the topology could be coarser then the one you want...
     
  4. radou

    radou 3,217
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    OK, but this seems very obvious, doesn't it? (unless I'm missing something, of course)

    In the metric topology, a set is open iff for every x in the set, there exists some open ball B(x, ε) contained in it. Take any integer N such that 1/N < ε. Then the open ball B(x, 1/N) is definitely contained in our open set.

    Hence, our collection is a basis "almost" by hypothesis.
     
  5. micromass

    micromass 18,702
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    It's a little trickier then that... You don't know that x is in A. Thus B(x,1/N) is not necessairily in the basis.
     
  6. radou

    radou 3,217
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    Hm, why would x be in A? Haven't I shown that my family covers X? Since I have shown that for any x in X there is some open ball containing it, and by definition, there must be an open ball around x contained in that very open ball, right?
     
  7. micromass

    micromass 18,702
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    Hmm, I'm just saying that if you take an open set G, then for every x in that open set, you need to find a basiselement B such that [tex]x\in B\subseteq G[/tex]. But if you take B=B(x,1/N), then this is not the correct choice since x doesnt have to A. And in that case B doesnt have to lie in our basis. So you'll need to find another candidate for B.

    I hope I didnt misunderstood your post...
     
  8. radou

    radou 3,217
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    Hm, OK, I think I get it now.

    Let U be an open subset of X, and let x be in U. Out family F is a cover for X, so take an element of the form B(x0, 1/n) containing x. Now we need to find an open ball around x itself contained in B(x0, 1/n), but if x is not in A...hmm...OK, I'll think about it some more.
     
  9. micromass

    micromass 18,702
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    You'll need to use density of A again, and probably the triangle inequality...
     
  10. radou

    radou 3,217
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    Just a small sub-question. The metric topology for a metric space X consists of all unions of elements of the family {B(x, ε) : x is in X and ε > 0}, right? This is true for any metric space, right?
     
  11. micromass

    micromass 18,702
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    Yes, that is true for any metric space!
     
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