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Especially with relation to topology - rigorous definitions are appreciated, but just as important is the intuition!

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(seeded question) –  BlueRaja - Danny Pflughoeft Jul 28 '10 at 16:04
    
This is relevant: mathoverflow.net/questions/19152/… –  BBischof Jul 28 '10 at 16:09
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You should make your question more specific. Are you just looking for definitions here? Or do you want some intuition... –  BBischof Jul 28 '10 at 16:10
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I think this is too vague of a question and is not a very good seeded question. If the questioner knows what a topology is, then they would be able to make the question more precise. If they don't know what a topology is, then they shouldn't be asking this question. –  Eric O. Korman Jul 28 '10 at 17:20
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The wording suggests that open and closed are mutually exclusive. Maybe you should ask "How is a set different from a door?" (no, actually, don't ask that either.) –  Tom Stephens Jul 28 '10 at 19:51

5 Answers 5

The rigorous definition of open and closed sets is fundamental to topology: you define a topology by saying what its open sets are. From this perspective, open and closed sets are axiomatic, like points and lines in geometry. In any case, closed sets are the complements of open sets and vice versa.

The most familiar example of open sets would be open intervals on the real line, intervals of the form {x : a < x < b}. Such sets and their arbitrary unions define the standard topology on the real line.

Note that the paragraph above describes the standard topology, but not the only one. You could put a very different topology on the real line. That's because a topology is determined by what you call open sets and not by the underlying space per se. For example, another topology on the real line defines a set to be open if its complement has only a finite number of points.

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Intuitively speaking, an open set is a set without a border: every element of the set has, in its neighborhood, other elements of the set. If, starting from a point of the open set, you move away a little, you never exit the set.

A closed set is the complement of an open set (i.e. what stays "outside" from the open set).

Note that some set exists, that are neither open nor closed.

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I think most sets are neither open nor closed, for some meaning of "most" (which I don't know). –  ShreevatsaR Jul 28 '10 at 20:14
    
it depends on the topology you are working with. If you work with the discrete topology... –  Soarer Jul 29 '10 at 7:12
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I thought there were some sets that are both open and closed, which is impossible by this definition. –  BlueRaja - Danny Pflughoeft Jul 20 '11 at 22:28
    
@BlueRaja-DannyPflughoeft No, on open set may very well have an open complement, which (the compliment) is then also closed, making it "clopen" as this is often called. –  fgp Oct 6 '12 at 1:21

A set X is open if for every point p in X, there exists a neighborhood (open ball) N of p such that N is a subset of X. We call the point p of the set X a limit point if every neighborhood of p has another point q which is also in X. The set X is closed if every limit point of X is a point of X. A set can be both open and closed, and such sets are occasionally termed "clopen." Trivial examples of clopen sets are the empty set (since it has no points, both the above definitions are vacuously true) and the set of all real numbers. You can in turn visualize an open set in R as an open interval on the real line, and a closed set as a closed interval on the real line.

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DOesn't your definition of closed set include all open sets? –  Isaac Jul 28 '10 at 17:00
    
I broke up the definition to be more clear. –  sordidarray Jul 28 '10 at 17:10
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I still think there's a problem--your definition of limit point p requires that p is of the set X, so every set is closed. –  Isaac Jul 28 '10 at 17:16

An open set is a set S for which, given any of its element A, you can find a ball centered in A and whose points are all in S.

A closed set is a set S for which, if you have a sequence of points in S who tend to a limit point B, B is also in S.

Intuitively, a closed set is a set which contains its own boundary, while an open set is a set where you are able not to leave it if you move just a little bit.

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*In a metric space. –  Qiaochu Yuan Jul 28 '10 at 17:38

I will not reiterate the very nice definitions found in the other answers, however I think that these "practical" definitions might help you as well on an intuitive level.

Open sets are typically used as domains for functions, as they are more useful for analysing "continuous" properties like differentiability. Also they don't have nasty borders (hence you don't have to deal with functions which are well behaved only on one side of the edge).

Closed sets are useful because, if they are limited, they are compact.

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