Defining Real Numbers Between Two Integers

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Discussion Overview

The discussion revolves around the concept of defining all real numbers between two integers through a process of iterative midpoint calculation. Participants explore whether this method can encompass irrational numbers as well as rational numbers, examining the implications of density and countability in the real number line.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant proposes that by marking midpoints between two integers and continuing this process indefinitely, it may be possible to define all real numbers, including irrationals, as limits of rational expressions.
  • Another participant counters that the resulting set would not be countable, suggesting a limitation in defining all real numbers through this method.
  • A third participant discusses the density of rational numbers in the interval between 0 and 1, arguing that the method can generate a sequence of rational numbers that are dense, thus implying that the construction can be extended to any interval between two integers.
  • Another viewpoint asserts that since the halfway point between any two rational numbers is also rational, the method cannot yield irrational numbers, leading to a conclusion that the approach is insufficient for defining all real numbers.

Areas of Agreement / Disagreement

Participants express differing views on the ability of the midpoint method to define all real numbers between two integers, with some arguing for its potential inclusivity of irrationals and others firmly stating that it cannot achieve this.

Contextual Notes

The discussion highlights assumptions regarding the nature of rational and irrational numbers, the concept of density in real numbers, and the implications of countability in the context of the proposed method.

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If we were to take any two integers on a real number line and mark a point (a number) halfway between the two, do the same in the range between the halfway point and each of the two numbers, and continue the process, would we be able to define all real numbers between the two integers (including irrational numbers) as the limit of a rational expression or a series of rational expressions?
 
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I think not. Because whatever you define would not be countable.
 
Consider in between 0 and 1. You'll end up getting every rational number that has a 2^k in the denominator. These points are dense, so every number between 0 and 1 is a limit of a sequence of them. Then given any two integers a and b, the map f(t)=a(1-t)+bt is a bijection between the intervals [0,1] and [a,b] that preserves your construction (so 1/2 is mapped to halfway between a and b, 1/4 is mapped to one quarter of the way between a and b) and so your construction is dense on any interval between two integers
 
I think the answer is no because the halfway point between any two rational numbers is another rational number. You can't obtain irrational numbers using this method.
 

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