Constructing a Perfect Set in R Without Rational Numbers

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Homework Help Overview

The discussion revolves around the existence of a nonempty perfect set in the real numbers that contains no rational numbers. The original poster seeks to understand how to construct such a set while adhering to the definition of a perfect set, which requires it to be closed and for every point to be a limit point.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss modifying the construction of the Cantor set to exclude rational numbers, questioning how to ensure that the remaining irrational numbers remain limit points without creating isolated points.

Discussion Status

There is an ongoing exploration of methods to construct the desired set, with some participants suggesting specific strategies, such as removing rational numbers at each step of the construction. The conversation reflects a mix of ideas and attempts to clarify the requirements for the set.

Contextual Notes

Participants note the challenge posed by the density of rational numbers in the real numbers, which complicates the construction of a perfect set that excludes them. There is also mention of the need to ensure that endpoints of segments remain irrational to maintain the properties of the set.

AbelAkil
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Homework Statement


Is there any nonempty perfect set in R which contains no rational number?


Homework Equations


A set E is perfect iff E is closed and every point of E is a limit point of E


The Attempt at a Solution


We should avoid rational numbers to become limit points, so we have to kick out countable segments with rational numbers...But how can I construct the set so that the remaining irrational numbers are still limit points and no isolated point exists?
 
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micromass said:
The construction of such a set is quite tricky. You will have to modify the construction of the Cantor set in such a way that it misses the rationals. See http://en.wikipedia.org/wiki/Smith–Volterra–Cantor_set for general information about such a set.

How to modify it? Rational numbers are dense in R...
 
Throw out a rational number at each step of the construction.
 
micromass said:
Throw out a rational number at each step of the construction.
yes, you are right...sorry, I made a mistake in my proof... but now I can understand it...thank U very much...
 
micromass said:
Throw out a rational number at each step of the construction.
We should choose the end points of each segments carefully to make sure that the end points are all irrational numbers and the set is still closed!
 

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