Thursday, January 21, 2016

elementary number theory - Proof that sqrt[3]17 is irrational



Consider 317. Like the famous proof that 2 is irrational, I also wish to prove that this number is irrational. Suppose it is rational, then we can write:



17=p3q3.


and then
17q3=p3




With the proof of 2 we used the fact that we got an even number at this step in the proof and that p and q were in lowest terms. However, 17 is a prime number, somehow we could use this fact and the fact that every number has a unique prime factorisation to arrive at a contradiction, but I don't quite see it yet.


Answer



The argument that works with 2 also works with 17. Since 17q3=p3, 17p3 and therefore 17p. Can you take it from here?


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