Heine-Porel from Bolzano-Weierstrass

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Homework Help Overview

The discussion revolves around proving the Heine-Borel theorem, which states that a subset of R^n is compact if and only if it is closed and bounded, using the Bolzano-Weierstrass theorem, which asserts that every bounded sequence in R^n has a convergent subsequence. Participants are exploring the relationship between these two theorems, particularly in the context of the unit interval [0, 1].

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Some participants attempt to clarify the definitions of compactness and continuity, questioning the original poster's use of "continuous interval" and suggesting "connected interval" instead. Others suggest starting with the assumption of a closed and bounded set and exploring how to relate the properties of the Bolzano-Weierstrass theorem to the definition of compactness.

Discussion Status

The discussion is active, with participants providing insights into the definitions and implications of the theorems. There is an ongoing exploration of how the properties of sequences in closed and bounded sets relate to compactness, and some participants are questioning assumptions and terminology used in the original post.

Contextual Notes

There is a mention of potential confusion regarding the terminology used, particularly the distinction between "continuous interval" and "connected interval." Additionally, one participant notes that the original poster may receive better responses in a different section of the forum.

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Homework Statement


How can you prove Heine-Porel (unit interval is compact) theorem by Bolzano-Weierstrass theorem (there is a limit in a continuous and bounded interval)?

The Attempt at a Solution


Compact means that the sequence is complete and totally bounded.

Unit interval perhaps means a bounded interval of an unit length.

I do not see how the continuity in Bolzano-Weierstrass theorem is related to
Heine-Porel theorem.
 
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Hi soopo - disclaimer first as I'm fairly new to this stuff myself

i think the Heine-Borel theorem actually states that a subsapce of Rn is compact iff it is closed & bounded

definition of compact is for every open cover, there exists a finite subcover
http://en.wikipedia.org/wiki/Open_cover

bolzano theorem states that each bounded sequence in Rn has a convergent subsequence.
http://en.wikipedia.org/wiki/Bolzano-Weierstrass_theorem

so start by assuming you have a closed & bounded set eg. [0,1]. Then try & show its compact... and then the other way.. and how can you relate the Bolzano conditions to the compactness cover definition & can you find a path between them?

by the way you would have a better chance of getting answered on the calculus part of this forum... Dick & HallsofIvy would rip through it
 
what "continuity" are you talking about? Bolzano-Weierstrasse says that every bounded sequence of real numbers contains a convergent subsequence. That says nothing about "continuity". I don't understand what you mean by "continuous interval"- it might be a language problem- "connected interval"?

Essentially, the proof that "Bolzano-Weierstrasse" implies "Heine-Borel" uses the fact that a sequence in a closed and bounded set (or, specifically, [0, 1]) is bounded because the set is bouded, so, by Bolzano-Weierstrasse, contains a convergent subsequence. The fact that the set is closed the implies that convergent subsequence converges to a point in the set and so Heine-Borel.
 
HallsofIvy said:
Essentially, the proof that "Bolzano-Weierstrasse" implies "Heine-Borel" uses the fact that a sequence in a closed and bounded set (or, specifically, [0, 1]) is bounded because the set is bouded, so, by Bolzano-Weierstrasse, contains a convergent subsequence. The fact that the set is closed the implies that convergent subsequence converges to a point in the set and so Heine-Borel.

The reason why B-W theorem implies Heine-Borel is that B-W tells us that a closed and bounded set (eg [0, 1]) contains a convergent subsequence.

HallsofIvy said:
I don't understand what you mean by "continuous interval"- it might be a language problem- "connected interval"?

I have been using a continuous interval as a synonym for a connected interval.
It seems that my convention is false.

Thank you for your reply!
 

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