Proving the Constructibility of Numbers: Hints and Proofs

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Discussion Overview

The discussion revolves around the concept of constructible numbers, specifically the equivalence between the definitions involving field operations and geometric constructions. Participants are exploring the proof of a theorem related to the constructibility of numbers, focusing on both directions of the equivalence.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes that a number is constructible if it can be obtained from the rationals by taking square roots a finite number of times or through a finite number of field operations.
  • Another participant questions the definition of a constructible number, suggesting that it involves the ability to construct a line segment of a given length using a compass and straightedge from a unit length segment.
  • A participant expresses confidence in proving the direction from the definition involving square roots to the geometric definition but struggles with proving the reverse direction.
  • Another participant introduces the idea that constructing numbers relates to finding intersection points of lines and circles, hinting at the underlying equations involved in such constructions.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the definitions of constructible numbers or the proofs of their equivalence. Multiple competing views on the definitions and approaches remain present in the discussion.

Contextual Notes

There are unresolved assumptions regarding the definitions of constructible numbers and the specific methods of proof being employed. The discussion also reflects varying interpretations of geometric constructions and their relationship to algebraic operations.

b0mb0nika
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Hi,

I was trying to prove the following theorem:

if x is a constructible number <=> it can be obtained from Q by taking a the square root a finite number of times ( or applying a finite # of field operations).

I managed to get the proof for <= this way, but I am not really sure on how to proceed to prove in the other direction =>.

Could anyone give me some hints/ proofs ?

thanks
 
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What is your definition of a constructible number? (Mine is precisely the one above that you're trying to prove is equivalent to yours.)
 
matt grime said:
What is your definition of a constructible number? (Mine is precisely the one above that you're trying to prove is equivalent to yours.)

This is what i thought the definition of a constructible number is :

A real number is constructible if and only if, given a line segment of unit length, one can construct a line segment of length | r | with compass and straightedge.

So then a line segment would be constructible ( by using the thm that i stated before) from Q ( as rational numbers are always constructible) by taking the sqrt a finitely # of times. ..

SO <= IF A=sqrt (a) ( a in Q) its easy to show that you can draw the length sqrt A. And we can extend this to taking the sqrt finitely many times.

I just don't know how to show that if A is constructible then A is egual to sqrt(sqrt(...(a) for some a in Q. ( finitely many sqrt's )
 
Constructing numbers amounts to looking at the points of intersection of various lines and circles, ie, the solutions of certain pairs of equations. What can you say about these equations?
 

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