Why Should We Consider Compactness in Sets?

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Discussion Overview

The discussion revolves around the importance of compactness in sets, particularly in the context of mathematical properties related to continuity, boundedness, and extrema. Participants explore the implications of compactness in various mathematical frameworks, including metric spaces.

Discussion Character

  • Exploratory, Conceptual clarification

Main Points Raised

  • One participant questions the significance of compact sets and notes their properties of being bounded and closed, as well as their favorable characteristics regarding continuity and extrema.
  • Another participant emphasizes that compactness can be seen as "the next best thing to finite," highlighting that compact sets share many properties with finite sets, such as being closed and bounded.
  • A further elaboration suggests that the proof of these properties can be approached through the concept of open covers and finite covers, linking compactness to finite sets.
  • Several participants express admiration for the succinct summary of compactness as "the next best thing to finite," indicating its resonance within the discussion.

Areas of Agreement / Disagreement

Participants generally agree on the significance of compactness and its properties, but the discussion remains exploratory without a definitive conclusion on its broader implications.

Contextual Notes

The discussion does not delve into specific mathematical proofs or definitions, leaving some assumptions and dependencies on definitions of compactness and related concepts unaddressed.

remaths
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Hi everyone,

I am asking very basic question. But I think there is some important concept I am missing.

why is compactness important or why should one look out for compact sets?

I understand that they are bounded and closed and functions on them have good properties wrt to continuity and maxima or minima.


thanks
remaths
 
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I've said this before but it bears repeating: compactness is "the next best thing to finite"! Consider all of the properties of a finite set: It is closed; it is bounded; in the case of a metric space, there exist two points in the set having maximum distance between them; in the case of a finite set of real numbers it contains a smallest and a largest number. All of those are true for a compact set. In fact, in each case you can prove them by taking an open cover consisting of open sets about each point in the set and using compactness to reduce to a finite cover- and so to a finite set of points.
 
Thank you for your reply
 
HallsofIvy said:
compactness is "the next best thing to finite"

This is one of the most amazing one-line summaries of a mathematical concept I have ever seen. I was about to say this about 100 times less eloquently.
 
halls beat me to it but i was going to parrot my analysis teacher and say exactly that
 

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