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Closed subset of a metric space

  1. May 17, 2007 #1
    This seems to be a very easy excercise, but I am completely stuck:
    Prove that in C([0,1]) with the metric
    [tex] \rho(f,g) = (\int_0^1|f(x)-g(x)|^2 dx)^{1/2} [/tex]

    a subset
    [tex]A = \{f \in C([0,1]); \int_0^1 f(x) dx = 0\}[/tex] is closed.

    I tried to show that the complement of A is open - it could be easily done if the metric was [tex] \rho(f,g) = sup_{x \in [0,1]}|f(x)-g(x)| [/tex] - but with the integral metric it's not that easy.

    Am I missing something?
    Thanks for any help.
    Last edited: May 17, 2007
  2. jcsd
  3. May 17, 2007 #2

    matt grime

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    I would attempt to show that the map f--->int f(x)dx is continuous.

    And you need / not \ in your closing tex tags.
  4. May 17, 2007 #3


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    Consider if:
    [tex]\left| \int_0^1 g(x)dx \right| = \epsilon[/tex]
    then you might be able to show that
    [tex]N_{\epsilon}(g(x)) \cap A = \emptyset[/tex]
    Last edited: May 17, 2007
  5. May 17, 2007 #4
    Well, I tried both, but the problem is that I still miss some kind of inequality that I could use.

    I mean - if I want to show the continuity for example - I have to show that:
    [tex] \forall \varepsilon > 0 \quad \exists \delta >0 \quad \forall g \in C([a,b]) : \rho(f,g)<\delta \quad |\int^1_0 f(x)-g(x) dx|< \varepsilon [/tex].

    But what then?
    [tex]|\int^1_0 f(x) - g(x) dx| \leq \int^1_0 |f(x) - g(x)| dx [/tex]
    but I miss some other inequality where I could compare it with [tex]\rho(f,g)[/tex]
  6. May 17, 2007 #5

    matt grime

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    Well, there is another inequality lying around. So use it That last inequality is also <=p(f,g).
  7. May 17, 2007 #6
    Of course :rolleyes: - a little modified Cauchy-Schwartz inequality is the key.
    I hate algebraic tricks :smile:
    Thanks for help
  8. May 18, 2007 #7

    matt grime

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    It's definitely not an algebraic trick. It is an application of the Jensen inequality from analysis. You might know it from probability theory, since it just states that the variance of a random variable is positive, i.e.


    where E is the expectation operator and X an r.v.
  9. Oct 30, 2010 #8
    How did you prove it for the supremum case? If you can prove it for the supremum, this proof here is self-contained because the given metric is always less than the supremum.
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