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Homework Help: Metric space completion

  1. May 1, 2007 #1
    1. The problem statement, all variables and given/known data

    Let X be an incomplete metric space. and Let X' denote its completion. I would like to show that there is Cauchy sequence in X which does not converge in X but does converge in X'. Moreover, I want to show that X contains every element of the sequence except the limit point.

    2. Relevant equations

    No equations as such other than the

    Definition: A complete metric space is one in which every Cauchy sequence converges.

    Definition: A Cauchy sequence { x_n } is one for which given any epsilon > 0 there exists a natural N such that if m,n > N then d(x_n,x_m) < epsilon.

    3. The attempt at a solution

    Well, negating the definition of a metric space we should be able to find a cauchy sequence which does not converge in X.

    My next line of argument would be to say that since X is isometrically embedded in X'. Then { x_n } is also a Cauchy sequence in X' and thus converges in X'.

    However, I'm not sure if it is always the case that X' \ X will always only contain the limit point. Can it not contain other points in the sequence?
    Last edited: May 1, 2007
  2. jcsd
  3. May 1, 2007 #2


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    I assume that was a typo and you mean "negating the definition of complete". Yes, since you are given that X is NOT complete, there must exist a Cauchy sequence in X which does not converge.

    Technically, there exist an isometric ((distance preserving), one-to-one function , f(xn)= x'n is in X'. Can you determine whether x'n is a Cauchy sequence in X'? If so, then since you know that X' is complete, the sequence {x'n} must converg to some point x'. Does there exist x in X such that f(x)= x'? If so what would be its relation to the sequence {xn}?
  4. May 1, 2007 #3
    Yes, we just choose the same natural number N since d(x_n,x_m) = d'(x_n',x_m').

    If there existed x in X such that f(x) = x'. Then d'(x', x_m') = d(x, x_m). But lim d(x,x_m) = lim d'(x',x_m') = 0 which would imply that { x_n } converges in X (contradiction).

    I'm still struggling to show that f(X) contains every element of the sequence except the limit point. Is this actually true?
  5. May 2, 2007 #4


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    What, exactly, is the definition of "completion" of a metric space?
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