Saturday, 11 July 2015

elementary number theory - Proving sqrt[3]2 is irrational without using prime factorization

Prove that 32 is irrational without using prime factorization.




The standard proof that 32 is irrational uses prime factorization in an essential way. So I wondered if there is a proof that does not use it.



This was inspired by the fact that I know two proofs that 2 is irrational that do not use prime factorization.



The first uses
2=22121=2221


to show that if 2=ab then
2=2abab1=2baab

is a rational 2 with a smaller denominator.



The second uses
(x22y2)2=(x2+2y2)22(2xy)2


and 32222=1 to show that x22y2=1 has arbitrarily large solutions and this contradicts 2 being rational.



I have not been able to extend either of these proofs to 32. Results that I do not consider "legal" in solving this problem include Fermat's Last Theorem (which definitely uses unique factorization) and the rational root theorem (which uses unique factorization
in its proof).

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