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

Every ultrafilter on a compact set converges. For example, a proof which comes from my head is: Sign up or log in Sign up using Google. Thank you for the compliment.

Sargera 2, 13 The rest of your example is very interesting and strong Compactness is the next best thing to finiteness. Please, could you detail more your point of view to me? In this topology we have less open sets which implies more compact sets and in particular, bounded sets are pre-compact sets. Compactness is important because: I think it’s a great example because it motivates the study of weaker notions of convergence.

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? It gives you the representation of analysls Borel measures as continuous linear functionals Iin Representation theorem.

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. Every compact Hausdorff space is normal.

Sign up using Facebook. 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. The condition of having finite subcover and finite refinement are equivalent. Every universal net in a compact set converges.


And when one learns about first order logic, gets the feeling that compactness is, somehow, deduce information about an “infinite” object by deducing it from its “finite” or from a finite number of parts.

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 space, define a notion of convergence.

If it helps answering, I compactnese about to enter my third year of my undergraduate hewtt, and came to wonder this upon preliminary reading of introductory topology, where I first found the definition of compactness. I can’t think of a good example to make this more precise now, though.

R K Sinha 4 6.

Consider the following Theorem:. Compactness does for continuous functions what finiteness does for functions in general.

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One reason is that boundedness doesn’t make sense in a general topological space. Mathematics Stack Exchange works best with JavaScript enabled. So why then compactness? The image of a compact space under a continuous function is compact. So I’m not sure this is a good example But for example, it gives you extremums when working with continuous functions on compact sets. I particularly like the phrase “finitely many possible behaviors”. Is there a redefinition of discrete so this principle works for all topological spaces e.

This list is far from over Every continuous bijection from a compact space to a Hausdorff space is a homeomorphism. This can be proved using topological compactness, or it can be proved using the completeness theorem: It seems like such a strange thing to define; why would the fact every open cover admits a finite refinement be so useful?

Kris 1, 8 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. For sequential compactness of a set, we ask: I would like to give here a example showing why compactness is important.


Well, finiteness allows us to construct things “by hand” and constructive results are a lot deeper, and to some extent useful to us. I was wondering if you had any nice examples that illustrate that first paragraph? This is throughout most of mathematics. Anyway, a topological space is finite iff it is both compact and P.

By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Compactenss of Service. A refinement is something different, used to define weaker related ideas. If you naalysis to understand the reasons for studying compactness, then looking at the reasons that it was invented, and the problems it was invented to solve, is one of the things you should do. Either way you look at it, though, the compactness theorem is a statement about the topological compactness of a particular space products of compact Stone spaces.

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 og compact spaces. A compact space looks finite on large scales. Compact spaces, being “pseudo-finite” in their nature are also well-behaved and we can prove interesting things about them.