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Toposym 1. Edwin Hewitt. Some applications to harmonic analysis, and so clearly illustrate the importance of compactness, that they should be cited. The first. This paper traces the history of compactness from the original motivating questions E. Hewitt, The role of compactness in analysis, Amer. Compactness. The importance of compactness in analysis is well known (see Munkres, p). In real anal- ysis, compactness is a relatively easy property to.

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A compact space looks finite on large scales. FireGarden, perhaps you are reading about paracompactness?

Because those are well-behaved properties, and we can control these constructions and prove interesting things about them. Not sure what this property P should be called Why is compactness so important? FireGarden 2, 2 15 This can be proved using topological compactness, or it can be proved using the completeness theorem: In every other respect, one could have used “discrete” in place of “compact”. Clark Sep 18 ’13 at As many have said, compactness is sort of a topological generalization of finiteness.

Every ultrafilter on a compact set converges. Moreover finite objects are well-behaved ones, so while compactness is not exactly finiteness, it does preserve a lot of this behavior because it behaves “like a finite set” for important topological properties and this means that we can actually work with compact spaces.

general topology – Why is compactness so important? – Mathematics Stack Exchange

It gives you convergent subsequences when working with arbitrary sequences that aren’t known to converge; the Arzela-Ascoli theorem is an important instance of this anaylsis the o of continuous functions this point of view is the basis for various “compactness” methods in the theory of non-linear PDE.


One reason is that boundedness doesn’t make sense in a general topological space. It gives you the representation of regular Borel measures as continuous linear functionals Riesz Representation theorem.

Essentially, compactness is “almost as good as” finiteness. This is throughout most of mathematics.

Consider the following Theorem: So why then compactness? Is there a redefinition of discrete so this principle works for all topological spaces e. A variation on that theme is to contrast compact spaces with discrete spaces. In this situation, for practical purposes, all I want to know about topologically for a given setting is, given a sequence of points in my compactnexs, define a notion of convergence.

The concept of a “coercive” function was unfamiliar to me until I read your answer; I suspect the same will be true for many readers. A locally compact abelian group is compact if and only if its Pontyagin dual is discrete. Historically, it led to the compactness theorem for first-order logic, but that’s over my head. Since there are a lot of theorems in real and complex analysis that uses Heine-Borel theorem, so the idea of compactness is too important.

Post as a guest Name. This relationship is a useful one because we now have a notion which is strongly related to boundedness which does generalise to topological spaces, unlike boundedness itself. Every infinite subset of a compact space has a limit point.

Every continuous bijection from a compact space to a Hausdorff space is a homeomorphism. Especially as stating “for every” open cover makes compactness a concept that must be very difficult thing to prove in general – what makes it worth the effort?

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If it helps answering, I am about to enter my third year of my undergraduate degree, and came to wonder this upon preliminary reading of introductory topology, where I first found the definition of compactness. So one way to think about compact sets in topological spaces is that they are analogous to the bounded sets in metric spaces.


Well, here are some facts that give equivalent definitions: By the way, as always, very nice to read your answers. Henrique Tyrrell 6 Please, could you detail more your point of view tbe me? Here are some more useful things: In probability they use the term “tightness” for measures Hmm.

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Simply put, compactness gives you something to work with, this “something” compactnses on the particular context. Anaysis list is far from over Post Your Answer Discard By clicking “Post Your Answer”, you acknowledge that you have read our updated terms of serviceprivacy policy and cookie policyand that your continued use of the website is subject to these policies.

If you have some object, then compactness allows you to extend results that you know are true for all finite sub-objects to the object itself. Mathematics Stack Exchange works best with JavaScript enabled.

Consider the following Theorem:. Compactness does for continuous functions what finiteness does for functions in general. Sargera 2, 13 Every net on a compact set has a convergent subnet. Sign up or log in Sign up using Google. The rest of your example is very interesting and strong By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

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