PlanetMath (more info)
 Math for the people, by the people. Sponsor PlanetMath
Encyclopedia | Requests | Forums | Docs | Wiki | Random | RSS  
Login
create new user
name:
pass:
forget your password?
Main Menu
Owner confidence rating: High Entry average rating: No information on entry rating
[parent] $\sqrt[n]{2}$ is irrational for $n\ge 3$ (proof using Fermat's last theorem) (Proof)
Theorem 1   If $n\ge 3$ , then $\sqrt[n]{2}$ is irrational.

The below proof can be seen as an example of a pathological proof. It gives no information to ``why" the result holds, or how non-trivial the result is. Yet, assuming Wiles' proof does not use the above theorem anywhere, it proves the statement. Otherwise, the below proof would be an example of a circular argument.

Proof. Suppose $\sqrt[n]{2}=a/b$ for some positive integers $a,b$ . It follows that $2=a^n/b^n$ , or \begin{eqnarray} \label{fermat} b^n + b^n &=& a^n. \end{eqnarray}We can now apply a recent result of Andrew Wiles [1], which states that there are no non-zero integers $a$ , $b$ satisfying equation ([*]). Thus $\sqrt[n]{2}$ is irrational. $ \qedsymbol$

The above proof is given in [2], where it is attributed to W.H. Schultz.

Bibliography

1
A. Wiles, Modular elliptic curves and Fermat's last theorem, Annals of Mathematics, Volume 141, No. 3 May, 1995, 443-551.
2
W.H. Schultz, An observation, American Mathematical Monthly, Vol. 110, Nr. 5, May 2003. (submitted by R. Ehrenborg).




Anyone with an account can edit this entry. Please help improve it!

"$\sqrt[n]{2}$ is irrational for $n\ge 3$ (proof using Fermat's last theorem)" is owned by matte. [ full author list (2) ]
(view preamble | get metadata)

View style:


This object's parent.
Log in to rate this entry.
(view current ratings)

Cross-references: equation, Andrew Wiles, integers, positive, circular argument, theorem, information, pathological, proof, irrational

This is version 10 of $\sqrt[n]{2}$ is irrational for $n\ge 3$ (proof using Fermat's last theorem), born on 2003-05-23, modified 2006-07-25.
Object id is 4292, canonical name is NthRootOf2IsIrrationalForNge3ProofUsingFermatsLastTheorem.
Accessed 2643 times total.

Classification:
AMS MSC11J72 (Number theory :: Diophantine approximation, transcendental number theory :: Irrationality; linear independence over a field)

Pending Errata and Addenda
None.
[ View all 3 ]
Discussion
Style: Expand: Order:
forum policy
circular? by rspuzio on 2005-06-07 02:55:20
Although this point does not affect the point you are getting across in the entry, it may affect its technical correctness. Are you sure that Wiles' proof does assume the result you are using it to prove? If so then this proof would be even more pathological --- it would be an example of circular reasoning! An argument based upon the properties of elliptic curves and modular functions assumes a fair amount of mathematics as background, so one would really need to check that the results used by Wiles, the results used to derive them, and so on down to the basics all do not depend on the fact about roots of 2. This would be a laborious and tedious undertaking for a human and so is a good argument for the usefulness of mathematical computer projects like Planet Math and the Hyperreal Dictionary of Mathematics.
[ reply | up ]

Interact
post | correct | update request | add example | add (any)