How to Prove a Nonempty Set of Real Numbers is Not Sequentially Compact?

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SUMMARY

The discussion centers on proving that a nonempty set of real numbers, denoted as S, is not sequentially compact. Two definitive approaches are outlined: (1) Assume S is bounded and demonstrate that there exists a sequence in S converging to a limit outside of S, or (2) Assume every convergent sequence in S has a limit within S and prove that S cannot be bounded. Both methods provide a structured pathway to establish the non-sequential compactness of S.

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  • Understanding of sequential compactness in topology
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  • Knowledge of bounded and unbounded sets
  • Basic principles of limits and continuity
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Suppose that S is a nonempty set of real numbers that is not Sequentially compact. Prove that either (i) there is an unbounded seqeunce in S or (ii) there is a sequence in S that converges to a point x0 that is not in S.

I am having trouble with this it not being sequentially compact is screwing me up, I don't know how to prove it.
 
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Well, there are two ways you can proceed:
(1) Suppose that S is bounded, and prove that there is a sequence in S that converges to something outside of S, or
(2) suppose that every convergent sequence in S has a limit in S, and prove that S cannot be bounded.

Have you tried either way? If so, what kind of problems did you run into?
 

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