What behind the idea of representing real numbers as points ?

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Discussion Overview

The discussion revolves around the representation of real numbers as points on a line and the underlying assumptions of this concept, particularly in relation to the Cartesian coordinate system. Participants explore the theoretical basis for this representation, its implications, and its extensions to other number systems.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the basis for representing real numbers as points on a line and seeks clarification on the assumptions involved.
  • Another participant suggests that the one-to-one correspondence between real numbers and points on a line stems from the completeness of the real number system, specifically mentioning Cauchy sequences.
  • A further participant agrees with the completeness argument but questions why a line is chosen for this representation.
  • One response highlights the utility of representing numbers geometrically to facilitate the interplay between algebra and geometry.
  • Another participant notes that the concept can be extended to complex numbers and their representation on a plane.
  • Contrasting views emerge, with one participant asserting that the correspondence is not necessarily true and is instead an axiom assumed without proof, referencing the Cantor–Dedekind axiom.
  • Some participants express uncertainty about the validity of the commonly accepted visualization of real numbers on a line.

Areas of Agreement / Disagreement

Participants express differing views on the assumptions underlying the representation of real numbers as points on a line. While some support the completeness argument, others challenge its validity and assert that it is an unproven axiom. The discussion remains unresolved regarding the foundational nature of this representation.

Contextual Notes

Participants highlight the dependence on definitions and the potential limitations of the assumptions involved in the correspondence between real numbers and points on a line. The discussion reflects varying interpretations of foundational concepts in analysis.

mahmoud2011
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I was wondering about the idea of representing real numbers as points on line , What is the basis of this assumptions , and as well the same question for Cartesian coordinates system ?
All books I have read , express the idea of Cartesian Coordinates in an elementary way like spivak's , Apostol , ... , many others that I have read this part in .

Thanks
 
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What "assumption" are you talking about? That there exist a one to one correspondence between real numbers and points on a line? That comes from the "completeness" of the real number system. one expression of which is that every Cauchy sequence converges.
 
HallsofIvy said:
What "assumption" are you talking about? That there exist a one to one correspondence between real numbers and points on a line? That comes from the "completeness" of the real number system. one expression of which is that every Cauchy sequence converges.

yes , that is what I knew , but why we choose line exactly
 
Why? So that we can "algebra-ize" geometry! And, "geometrize" algebra. It is often easier to visualize geometry than algebra, easier to get precise values for algebra than geometry. To be able to convert from one to the other helps both ways.
 
And the same concept can be extended easily to complex numbers and points on a plane.
 
so my concept is ok , I thought I have something missing .

Thanks
 
HallsofIvy said:
What "assumption" are you talking about? That there exist a one to one correspondence between real numbers and points on a line? That comes from the "completeness" of the real number system. one expression of which is that every Cauchy sequence converges.

I don't believe that's correct. The correspondence between the real numbers as defined in analysis, on the one hand, and the geometrical line on the other, is not necessarily true. It's an axiom; that is, it's assumed without proof.

http://en.wikipedia.org/wiki/Cantor–Dedekind_axiom

We use this visualization so often that we accept it as necessarily true; but it's not.

There's been some discussion of this on PF.

https://www.physicsforums.com/showthread.php?t=244274
 

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