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Homework Help: Proof about cover

  1. Jul 5, 2013 #1
    1. The problem statement, all variables and given/known data
    (0,100) has a cover that consists of a finite number of closed interval subsets.

    I'm really lost with this one. I can clearly understand why the statement is false, but I'm not sure my proof is good.

    2. Relevant equations

    3. The attempt at a solution
    Clearly this is false, so I am trying to disprove it.

    Let S =(0,100) and let C be a cover of S.
    Since C contains finitely many closed interval subsets of S,
    C has a least element.
    Let X_i =[a,b][itex]\in[/itex]C be a subset of S, [itex]\forall[/itex]a,b[itex]\in[/itex]R^+
    such that 0<a<b<100

    Since there is no smallest positive real number, (which i have proved before),
    there is an infinite number of X_i's in C.
    But C has a finite number of elements.
    This is a contradiction.
  2. jcsd
  3. Jul 5, 2013 #2


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    What does it mean for C to have a least element? Can one have a least set?

    Stuff like this is the problem you have, things that are meaningless or not properly defined.
  4. Jul 5, 2013 #3
    It certainly seems like it. If C is a set containing a finite number of sets, then you could number each set in C, and since there is a finite number, one of them must be the least?
  5. Jul 5, 2013 #4
    Well, after considering what you said a little more...

    I was thinking that I could let C be the cover of (0,100), then come up with some sort of closed interval like

    [50-v(n), 50+v(n)]

    to represent a generic closed interval subset in C.
    v(n) in this case, would be a function of real number n, such that as n approaches infinity,
    the interval converges on [0,100]
    (If this is even possible to write, I haven't came up with a v(n) yet).

    If the cover is a set of many such [50-v(n), 50+v(n)] 's with different n's, then could I show that there is always another n value (and thus, another set in C), which would bring the interval closer to (0,1).

    Which means there would NEED to be an infinite number of closed-interval subsets in C, and thus, there is a contradiction.

    My thought is, that this is not valid because then it only shows that C contains infinitely many closed interval subsets of (0,100) for that specific type of closed interval, and not "any". Since there could be many different covers...

    Am I even on the right path here? When I try to look up more information that would be helpful, like that "The union of finitely many closed intervals is closed", I only find information on Topology which is far outside the scope of my class.

    I feel that there is just something really obvious that I am missing here.
  6. Jul 5, 2013 #5


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    If by 'cover' you mean there are a finite number of sets whose union equals (0,100), then I think your original idea for the proof works fine. If ##X_i=[a_i,b_i]## take the minimum of the ##a_i##.
  7. Jul 6, 2013 #6
    I think I ended up doing something like that.

    My proof was basically this (I'm paraphrasing from memory)

    Assume that (0,100) has a cover consisting of a finite number of closed interval subsets.
    Let S be a collection of closed interval subsets of (0,100), such that
    [itex](0,100)\in\bigcup_{i=1}^{n}S_{i}[/itex] So, it's a cover of (0,100)...

    We organize the closed intervals in S first by left bound values in ascending order, then by right bounds in ascending order. Resulting in the ordered S having the form:
    [itex]S = {[a_1,b_1],[a_1,b_2],...,[a_1,b_j],[a_2,b_1],[a_2,b_2],...,[a_2,b_j],...,[a_i,b_{j-i}],[a_i,b_j]}[/itex] for i,j are natural numbers.

    So each closed interval subset of (0,100) in S, is represented as [itex][a_1, b_j][/itex]

    Then I noted for clarification, that i and j are simply indexes, and that each a_i and b_j were in fact real numbers.

    Now I defined a value that I called L, the lowest value of S. (This is just my convention for this).
    L is the smallest of either [itex]a_1[/itex] or [itex]b_1[/itex],
    if they are equal (then the interval is a closed singleton?), then I just said "take a_1 then"...

    so L is now basically defined as the smallest number like Dick mentioned above.

    Then I claimed that for any L in (0,1), there is a K in (0,1) such that K = L/2.

    This shows that S has infinitely many elements, and thus, is contradictory to my assumption that it had a finite number of elements.

    How does that look?
    Last edited: Jul 6, 2013
  8. Jul 6, 2013 #7


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    Your set of closed intervals is {[a1,b1],[a2,b2],[a3,b3],...,[an,bn]}. That doesn't include intervals like [a1,b2]. And in the notation [ai,bi] it's always the case that ai<=bi. If you clear up some confusion about the notation the proof is a lot simpler.
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