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Homework Help: Upper and lower limit proof (liminf/sup)

  1. Sep 3, 2012 #1
    So I have been working on this question for quite a while now and I'm at this point.
    I just wanted to check if everything was okay, I never feel confident with questions like this. Here is the question and my working,

    http://img404.imageshack.us/img404/8436/afafafa.jpg [Broken]

    Is there anything wrong?

    Thanks in advanced.
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Sep 3, 2012 #2
    Are there are conditions on Xn and Yn? The inequality is not true in general. If we set Xn - Yn = a = const, then Xn + Yn = 2, so lim inf {Xn + Yn} = lim sup {Xn + Yn} = a, but lim inf Xn and lim sup Xy may not exist.
    Last edited: Sep 3, 2012
  4. Sep 3, 2012 #3
    The conditions are that they are both bounded real sequences,

  5. Sep 3, 2012 #4
    OK, bounded makes it correct.

    You have the first part correctly.

    The second, however, is wrong. You show that Ix + Sy <= Sx + Sy. That's true, but that does not mean that Ix + Sx <= Sw.
  6. Sep 4, 2012 #5
    I have found another way, do you think this works?

    http://img211.imageshack.us/img211/1769/89852645.jpg [Broken]

    the last part of the first line sup(inf(x)+y) = inf(x) + sup(y), because inf(x) will just be some number,
    Last edited by a moderator: May 6, 2017
  7. Sep 4, 2012 #6
    This looks good to me.
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