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Sequence question

  • Thread starter happyg1
  • Start date
  • #1
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Hi,
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.
CC
 

Answers and Replies

  • #2
AKG
Science Advisor
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{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.
 
  • #3
308
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Hi,
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.
Thanks
CC
 
  • #4
HallsofIvy
Science Advisor
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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.
 
  • #5
308
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Our definition of oscillating is a sequence that diverges but not to + infinity or - infinity.
 

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