An Interval that is both open and closed.

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Homework Help Overview

The discussion revolves around the properties of intervals in the real numbers, specifically exploring the conditions under which an interval can be both open and closed simultaneously. Participants are tasked with proving that such an interval must either be the entire set of real numbers or the empty set.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are examining the definitions of open and closed sets, questioning the implications of a set being both. There is an exploration of boundary points and their relationship to the properties of the set in question. Some participants are considering contradictions based on assumptions about the nature of the interval.

Discussion Status

The discussion is active, with participants providing insights into the definitions of open and closed sets and their boundary points. There is a productive exchange of ideas regarding the implications of having no boundary points and how that relates to the original problem statement.

Contextual Notes

Participants are operating under the assumption that the interval is non-empty and are exploring the consequences of this assumption. The nature of boundary points is under scrutiny, particularly in relation to the empty set and the set of all real numbers.

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Homework Statement


Let I C R be an interval which is open and closed at the same time. Prove that I=R or I is the empty set.


Homework Equations





The Attempt at a Solution



I'm looking more for a outline structure for the solution. I have made assumptions that I is not equal to R seeking a contradition, but i don't know what further assumptions to make.
 
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A closed set is one that contains all of its boundary points. An open set is one that contains none of its boundary points.

In order to be both open and closed, a set must contain all of its boundary points and none of its boundary points!

If "all boundary points" is the same as "no boundary points", what can you say about the boundary points of the set?
 
Well, the boundary point can't be real.., maybe i can say that the set is unbounded
 
If a set has NO boundary points the "all boundary points" is the same as "no boundary points". A set is both open and closed if and only if it has no boundary points. What are the boundary points of the empty set? What are the boundary points of R?

(A point, p, is a boundary point of set, S, if and only if every neighborhood of p contains points of both A and the complement of S.)
 

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