Analysis Help: Limit Points of a Sequence

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    Analysis Limit Points
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Discussion Overview

The discussion centers around finding the limit points of the sequence defined by the set A = {((-1)^n)(2n/(n-4))}. Participants explore the nature of limit points, isolated points, and the convergence behavior of the sequence.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that 0 is a limit point of the set A but is uncertain if there are additional limit points.
  • Another participant questions the reasoning behind considering 0 as a limit point and prompts a discussion about the behavior of |An| as n approaches infinity.
  • A participant reflects on the limit of the sequence, initially thinking it approaches 0 but later considering that it might actually approach 2, leading to the conclusion that the limit points could be -2 and 2 due to the alternating nature of the sequence.
  • Another participant clarifies that the sequence 2n/(n-4) converges to 2, while the sequence defined by A does not converge and has two convergent subsequences, leading to the conclusion that the set of limit points is {-2, 2}.

Areas of Agreement / Disagreement

Participants express differing views on the limit points of the sequence, with some suggesting 0 and others asserting that the limit points are -2 and 2. The discussion remains unresolved regarding the characterization of limit points and the behavior of the sequence.

Contextual Notes

There are unresolved assumptions regarding the definitions of limit points and isolated points, as well as the convergence behavior of the sequence. The discussion also reflects varying interpretations of the sequence's limits.

Who May Find This Useful

Readers interested in mathematical analysis, particularly in the study of sequences and limit points, may find this discussion relevant.

Armbru35
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Hi, I am asked to find the limit points of the given set A

A={((-1)^n)(2n/(n-4))}

so far I think 0 is a limit point, but I can't figure out if there are any more than just that one.

I believe every point in A is an isolated point, and that this set is open because every point in A contains a neighborhood contained in the set.

Any suggestions would be helpful. Thanks!
 
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Armbru35 said:
so far I think 0 is a limit point,
Why do you think that?
Consider |An|. What happens as n goes to infinity?
I believe every point in A is an isolated point, and that this set is open
If a point is isolated, how can there be an open neighborhood?
 
I guess I thought the limit of infinity/infinity would be 0, but now that I look at it more I feel like the limit would be 2, and because we have the (-1) in front, would the limits be -2 and 2?
 
You are using the word "limit" very carelessly. The sequence 2n/(n-4) has limit 2. That means its "set of limit points" is {2}. The sequence [itex](-1)^n2n/(n-4)[/itex] does not converge- it has no limit. It does have two convergent subsequences- taking only even indices, it converges to 2, taking only odd indicees it converges to -2. So its "set of limits points" (not "its limits") is {-2, 2}.
 

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