Show sequence is bounded in an interval

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Homework Help Overview

The discussion revolves around proving that a sequence {a_n} is bounded if and only if there exists an interval [c, d] such that the sequence lies within this interval. The participants are examining the definitions and implications of bounded sequences in the context of real analysis.

Discussion Character

  • Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants explore the implications of defining the interval [c, d] and question the validity of certain assumptions, such as setting c equal to -d. There is also discussion about how to express the bounds of the sequence in relation to M.

Discussion Status

Some participants have provided guidance on how to approach the proof, suggesting that defining M as max{|c|, |d|} could be a viable step. However, there remains uncertainty about how explicitly to present the proof, indicating that further clarification may be needed.

Contextual Notes

There is a mention of potential assumptions regarding the relationship between c and d, as well as the need to express the proof in a clear manner. The discussion reflects a collaborative effort to refine the proof without reaching a definitive conclusion.

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Homework Statement



Show that a sequence {a_n} is bounded if and only if there is an interval [c, d] such that {a_n} is a sequence in [c,d].

Homework Equations



A sequence {a_n} is bounded provided that there is a number M such that |a_n| <= M.

The Attempt at a Solution



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The first part looks fine. I'm having difficulty reading the second part, but it looks like you assume the c=-d, which isn't necessarily a valid assumption.
 
For the second part I let c = -d because I was trying to end with a result such as the sequence being in the interval [-M, M]. If I leave it as the interval [c, d] then I'm not sure how to end with the conclusion that every element of the sequence is less than a number M.
 
Consider max{|c|,|d|}.
 
So I can define M to be max{ |c|, |d| }.

Therefore, |a_n| <= M.

Would this be all that I need to write for the proof in that direction?
 
It might be nice if you write things out a little more explicitly, but yes, it's all that you need.
 
I'm not really sure how I should be more explicit for the proof.
 
[tex]-\max\{|c|,|d|\} \leq c \leq a_n \leq d \leq \max\{|c|,|d|\}[/tex]
 

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