Proof or Counterexaple that a_n and b_n converge

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SUMMARY

The discussion centers on the convergence of sequences a_n and b_n, specifically addressing the claim that if both a_n + b_n and a_n - b_n converge, then a_n and b_n must also converge. Participants explore potential counterexamples, testing sequences such as a_n = (-1)^n and b_n = 1/n, but fail to find a valid counterexample. The conclusion suggests that while the initial assumption may seem plausible, further investigation and proof are necessary to confirm the convergence of a_n and b_n.

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Homework Statement



If a_n+b_n and a_n-b_n converge then a_n and b_n must converge.
This MAY be a counterexample problem.

Homework Equations



N/A

The Attempt at a Solution



I have tried the following possibilities out of a_n and b_n and none of these give a counterexample:
Let a_n and b_n= (-1)^n , 1/n , 1/n^2 , (-1)^2n , sqrt(n)
I need to come up with something similar to this that holds true that a_n+b_n converge, a_n-b_n converge, but a_n and b_n alone do not converge.
 
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Nana said:

Homework Statement



If a_n+b_n and a_n-b_n converge then a_n and b_n must converge.
This is going to be a counterexample problem.

Homework Equations



N/A

The Attempt at a Solution



I have tried the following possibilities out of a_n and b_n and none of these give a counterexample:
Let a_n and b_n= (-1)^n , 1/n , 1/n^2 , (-1)^2n , sqrt(n)
I need to come up with something similar to this that holds true that a_n+b_n converge, a_n-b_n converge, but a_n and b_n alone do not converge.

How do you know there is a counterexample?
 
Why are you so sure there is a counterexample?
 
LCKurtz said:
How do you know there is a counterexample?

We have been doing similar problems and they were all counterexamples, but there is a possibility that it is not.
 
LCKurtz said:
How do you know there is a counterexample?

PeroK said:
Why are you so sure there is a counterexample?

It may not actually be. Thanks, I have updated my question. We were doing lots of counterexamples and it led me to believe that was one of them.
 
You should try proving it. Because it is true.
 
Hint: the two rows converge for n>N and for n>M respectively, what happens if you look at n>(N+M) and add the two sequences?
 

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