Understanding Infinite Limits: A Formal Definition and Explanation

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SUMMARY

The limit of a sequence \(X_n\) as \(n\) approaches infinity being equal to infinity indicates that the sequence is unbounded. The traditional epsilon-delta definition of limits, which focuses on approaching a specific value, does not apply in this case. Instead, one should consider the concept of "arbitrary largeness" of \(X_n\) as \(n\) increases. A formal definition can be articulated by stating that for any arbitrary large number \(N\), there exists an \(n\) such that \(X_n > N\).

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  • Understanding of limits in calculus
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  • Basic knowledge of sequences and series
  • Concept of bounded vs. unbounded sequences
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Homework Statement



Give a definition to explain what it means if the limit of Xn as n->infinity is itself equal to infinity.

Homework Equations





The Attempt at a Solution



It seems the typical use of epsilon to show limits (that we approach arbitrary closeness) doesn't work here, so I had thought I could say it is the opposite of being bounded, but I'm not sure about how to say this more formally...

Thanks for any ideas you may be able to offer
 
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tony979 said:

It seems the typical use of epsilon to show limits (that we approach arbitrary closeness) doesn't work here, so I had thought I could say it is the opposite of being bounded, but I'm not sure about how to say this more formally...


Well you mention the idea of "arbitrary closeness" using the epsilon definition. Why not think about it in terms of the arbitrary "largeness" of xn as n becomes large?
Or try to use epsilon in reverse?
 
The "the typical use of epsilon to show limits " works fine. It is the "delta" that doesn't work! Saying that a number is "going to infinity" does mean that it is unbounded so instead of "|x- a|< \delta" as you would with "limit as x goes to a", how about something like "x> N" for some number N.
 

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