Lim sup an /bn = lim sup an / lim sup bn?

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Discussion Overview

The discussion revolves around the properties of limits superior (lim sup) in the context of sequences, specifically addressing whether the relationship lim sup an / bn equals lim sup an / lim sup bn holds true, and whether the finiteness of lim sup an / bn implies that the sequence {an/bn} is bounded.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that lim sup an / bn does not equal lim sup an / lim sup bn, providing a counterexample with sequences (an) and (bn) defined as (0,1,0,1,...) and (1,-1,1,-1,...).
  • One participant believes that if lim sup an / bn is finite, then the sequence {an/bn} is bounded, but this claim is challenged by another participant.
  • Another participant argues that the claim about boundedness is false, citing a scenario where bn equals zero for a finite number of n's, which raises concerns about the definition of the sequence an/bn.
  • A participant clarifies that they meant bn to be strictly positive, indicating a specific condition for the discussion.
  • Further exploration of the boundedness question is presented, with an example where bn is constant and an takes on values that lead to a finite lim sup an / bn, yet the sequence an/bn is not bounded from below.

Areas of Agreement / Disagreement

Participants express disagreement regarding the relationship between lim sup an / bn and lim sup an / lim sup bn, as well as the implications of a finite lim sup an / bn on the boundedness of the sequence {an/bn}. The discussion remains unresolved with multiple competing views.

Contextual Notes

Participants have not fully defined the conditions under which the sequences are considered, particularly regarding the positivity of bn and the implications of its values on the sequence an/bn.

jem05
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hello,
2 technicality questions:

1) lim sup an /bn = lim sup an / lim sup bn?
2) if lim sup an /bn is finite does that mean
that the sequence {an/bn} bounded?

thank you.
 
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1) no. Take (an)=(0,1,0,1,0,1,...) and (bn)=(1,-1,1,-1,1,-1,...).
2) I believe that this is true...
 
2) is false. Let bn=0 for a finite number of n's.
 
Uh, if a bn=0, then the sequence a_n/b_n isn't even good defined. I didnt think the OP meant that...
 
yes ofcourse, i meant bn strictly positive.
ok, thanks a lot
 
jem05 said:
2) if lim sup an /bn is finite does that mean that the sequence {an/bn} bounded?

It depends what "bounded" means.

For example, take bn = 1 for every n, take an = 1/n if n is odd, and take an = -n if n is even.

Then, if I'm thinking about this correctly, lim sup an /bn = 0, but an /bn is not bounded from below.
 

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