Topology intervals on the real line proof

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Homework Help Overview

The discussion revolves around proving properties of intervals on the real line. The original poster presents two statements regarding intervals: the first concerns the definition of an interval in terms of containing all points between any two of its points, and the second involves showing that the union of a collection of non-empty intersecting intervals is also an interval.

Discussion Character

  • Conceptual clarification, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants explore the implications of the betweenness property for defining intervals. There is a discussion about the necessity of proving both directions of the 'if and only if' statement in part (a). Some participants question how to define endpoints of a set and whether the proof needs to account for infinite intervals.

Discussion Status

Participants are actively engaging with the definitions and properties of intervals, with some providing insights into the proof structure while others raise questions about assumptions and the completeness of the arguments presented. There is recognition that clarity and thoroughness are needed in the proof, especially regarding the existence of endpoints.

Contextual Notes

There is an acknowledgment that the original poster's set I may not have a smallest or largest element, which introduces the possibility of infinite intervals. This aspect is under consideration as participants discuss the implications for the proof.

hlin818
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Homework Statement



a) Let I be a subset of the real line. Prove I is an interval if and only if it contains each point between any two of its points.

b) Let Ia be a collection of intervals on the real line such that the intersection of the collection is nonempty. Show the union of the collection is an interval.

Homework Equations





The Attempt at a Solution



a) An interval is the set (a,b), [a,b], or [a,b). Let I be one of these sets. we want to show that if x and z are in I and x<y<z, then y is in I. Isn't this trivial by how interval is defined?

b) not sure
 
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Yes, if I is an interval then the proof of the property is trivial. But you want to show 'iff'. Now you want to show that the betweeness property of I implies I is an interval.
 
Ah forgot about the other direction. Let some subset of R be I such that I contains each point between any two of its points. Thus if x,z are in I and x<y<z, then y is in I. Let x,z be the endpoints of I. Thus by the betweenness property any y such that x<y<z implies y is in I. So I is an interval.

b)
 
Last edited:
hlin818 said:
Ah forgot about the other direction. Let some subset of R be I such that I contains each point between any two of its points. Thus if x,z are in I and x<y<z, then y is in I. Let x,z be the endpoints of I. Thus by the betweenness property any y such that x<y<z implies y is in I. So I is an interval.

b)

I is just a set until you show it's an interval. How do you define endpoints of a set?
 
Dick said:
I is just a set until you show it's an interval. How do you define endpoints of a set?

I suppose by z as the right endpoint if z is greater than or equal to all y in I and likewise for x. So if we define x to be the smallest element in I and z to be the largest, by the betweenness property we have that I contains every point in between x and z, i.e. for any x<y<z, y is in I. Doesnt that mean I is an interval?
 
Last edited:
hlin818 said:
I suppose by z as the right endpoint if z is greater than or equal to all y in I and likewise for x. So if we define x to be the smallest element in I and z to be the largest, by the betweenness property we have that I contains every point in between x and z, i.e. for any x<y<z, y is in I. Doesnt that mean I is an interval?

Sure it does, but you have to spell all that out in a proof. There's also the possibility that I may not have a smallest or largest element which means you should include the possibility of infinite intervals. I'm not saying the proof isn't easy. You just have to be more explicit about the argument.
 

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