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Supremum and Infimum graphs

  1. Mar 8, 2012 #1
    In class, we have been introduced to the supremum and infimum concepts and shown them on graphs, but I am wondering how to go about deriving them, and determining if they are part of the set, without actually having to graph them- especially for more complicated sets.
  2. jcsd
  3. Mar 8, 2012 #2


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    Are you able to find "upper bound" and "lower bounds" for sets?
  4. Mar 8, 2012 #3


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    Some ideas:

    Have you already seen limit points? If so, try showing that both the sup and the inf (when both are finite*) , are limit points of a set. Then look for a charcterization of closed sets in terms of limit points.

    *This can be extended to the infinite case too, but let's start slowly.
  5. Mar 9, 2012 #4
    How would you go about extending it to infinity?

    In the text it has a few examples that span n from 1 to infinity.

    Such as, an=n(-1)^n

    I understand that it does converge, because an approaches 0 as n approaches infinity, but when the equations become more complicated, how to I recognise this without a graph?
  6. Mar 9, 2012 #5


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    I actually used 'limit point' here a little too losely (specially since ∞ is not a real number); what I meant is that , in the case the sup is ∞ , the values would become indefinitely-large. In the extended reals, every 'hood (neighborhood) of ∞ would contain points of the set.

    To recognize/determine the limit, I would suggest looking at the expression and trying to understand what happens with it as you approach ∞. Does it oscillate, increase, etc. If you cannot tell right away, consider trial-and-error. Assume a certain value is the Sup (Inf) , and put it to the test. That is the best I got; I cannot think of any sort of algorithm. It just seems to come down to practicing.
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