Sunday, 9 December 2018

elementary number theory - Prove that if x9equiv3mod61, then x12equiv34mod61



Prove that if x^9 \equiv 3 \mod{61}, then x^{12}\equiv 34 \mod{61}.




The first thing I tried was the obvious:



(x^9)^{4/3} \equiv 3^{4/3} \mod{61}
x^{12} \equiv 3^{4/3} \mod{61}



But this doesn't get me anywhere.



I've also tried breaking it down by saying that there exists a k_1 \in \mathbb{Z} s.t. x^9 - 3 = 61k_1 and trying to work it towards x^{12} - 34 = 61k_2, for some k_2 \in \mathbb{Z}, but again, I didn't get too far.


Answer



Recall that x^{60} = 1 modulo 61. This is Fermats Little Theorem (of course x is a not divisible by 61 so it is applicable).




Thus, 3^7 = (x^9)^7 = x^{63}=x^3 modulo 61.
So you know x^3 and the rest is direct.



The general strategy would be to solve 9 k = 12 modulo 60, and then compute x^{12}= (x^{9})^k = 3^k, but I took a slight shortcut.


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