Show that the nested intervals property fails for the rational numbers

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Homework Help Overview

The discussion revolves around demonstrating that the nested intervals property fails for the rational numbers by constructing a sequence of closed, bounded intervals whose intersection is empty. The problem context involves concepts from real analysis and topology, particularly focusing on the properties of intervals in the rational number field.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants suggest centering intervals around irrational numbers, particularly using radical 2, and discuss the implications of interval lengths decreasing. Questions are raised about the topology assumed for the rational numbers and the nature of intervals within this context.

Discussion Status

The discussion is active, with participants exploring different interpretations of the problem and clarifying the nature of intervals in the rational field. Some guidance has been offered regarding the construction of intervals, but no consensus has been reached on the assumptions or definitions involved.

Contextual Notes

There is uncertainty regarding the topology assumed for the rational numbers and how intervals are defined in this context. Additionally, the distinction between closed and open intervals in relation to irrational endpoints is noted.

Kb13
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In the field of formal rational functions, construct a nest of closed, bounded intervals whose intersection is empty. (That is, show that the Nested Intervals property fails in this field)


I know it has to involve radical 2 but because that is the only number we know is irrational but other than that I have no idea.
 
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Try 'centering' your intervals around an irrational (you should know more irrationals than that, but sqrt2 is still a good choice). For example, one interval can be [sqrt2 - 1/2, sqrt2 + 1/2]. Turn this into a nested sequence and consider what happens as the length of the intervals get very small.
 
Kb13 said:
In the field of formal rational functions, construct a nest of closed, bounded intervals whose intersection is empty. (That is, show that the Nested Intervals property fails in this field)

Is there a topology assumed for this field? If so, what is it? I'm not sure what an "interval" looks like in this field.
 
I suspected he meant the set of rational numbers (since this is a common problem and he didn't know any other irrationals so odds are he doesn't know what a topology is).
 
Good point. I would like to point out one subtlety that he might otherwise overlook, namely that [sqrt2 - 1/2, sqrt2 + 1/2] in the rational field does NOT contain the endpoints since sqrt2 +/- 1/2 is irrational. It is nonetheless a closed (AND an open) interval.
 

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