Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Sequence question

  1. Sep 26, 2005 #1
    Here's the question:
    If {sn} is oscillating and not bounded, and {tn} is bounded, then {sn+tn} is oscillating and not bounded.
    True or False?

    I have tried for a while to find a counterexample, but I can't. I am leaning towards saying that this is true....but then I have to prove it. Am I correct in saying that it's true?
    I have also started trying to prove it, but I'm having a hard time. Any clarification or advice will be appreciated.
  2. jcsd
  3. Sep 26, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper

    {sn} is not bounded, and let's say that it's not bounded above in particular. Let t be inf{tn}. Then (sn + tn) > (sn + t) for all n, so if {sn + t} is not bounded above, then neither is {sn + tn}. But if {sn + t} is bounded, then there is some K such that sn + t < K for all n, so sn < K - t for all n, so {sn} is bounded above, contradiction.
  4. Sep 27, 2005 #3
    Thanks for the help. I was trying to prove it directly and I wound up with a bunch of nonsense. I understand the problem now, and I have worked the other 3 that were giving me headaches.
  5. Sep 27, 2005 #4


    User Avatar
    Science Advisor

    What, precisely, do you mean by "oscillating"? I have seen it used to mean that terms alternate sign or simply to mean that sn-1< sn but sn> sn+1.
  6. Oct 6, 2005 #5
    Our definition of oscillating is a sequence that diverges but not to + infinity or - infinity.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook