Monday, November 6, 2017

modular arithmetic - How to solve congruence xy=apmodp?

I'm having trouble solving this congruence:
x^{114} \equiv 13 \pmod {29}.
I thought that it made sense to try to solve it using this idea: "Suppose you want to solve the congruence x^y \equiv a \pmod p (we will assume for the moment
that p is prime). Raise both sides to the power z to obtain x^{yz} \equiv a^z \pmod p. Now if we can
find a z such that yz \equiv 1 \pmod {p − 1} then the solution of the congruence will be x \equiv a^z \pmod p."




So I set 114 z \equiv 1 \pmod {28}. However, \gcd(114, 28) = 2 and I can't solve for the inverse using the Euclidean algorithm. Does that statement that I quoted even come in handy anywhere?



Next I simplified x^{114} to x^2 by Fermat's theorem.



I know that x^2 \equiv 13 \pmod {29} has a solution because of the Legendre symbol:
\left(\frac{13}{29}\right) \equiv 13^{14} \pmod {29} = 1



The only way I have learned to solve for square roots is when p \equiv 3 \pmod 4. Since this isn't the case, I'm at a loss as to how to find the square root. Any tips or hints?

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