Let $p, q$ positive prime numbers, $q > 5$. Prove that if
$q \mid \left(2^{p} + 3^{p}\right)$ then $q > p$.
First, it's clear that $p \ne q$ because, using Fermat's little theorem, $2^p = 2 \mod p$ and $3^p = 3 \mod p$, therefore $5=0 \mod p$ false because $q \gt 5$.
Now suppose $q \lt p$. I tried to analyse $2^p +3^p \mod q$ but I didn't get a contradiction.
Answer
HINT:
For $p>2$
$$\left(-\dfrac32\right)^p\equiv1\pmod q$$
$\implies$ord$_q(-3/2)|(p,q-1)$
But As $p$ is prime, ord$_q(-3/2)=1$ or $p$
ord$_q(-3/2)=1\implies-3\equiv2\pmod q\iff5\equiv0\iff q|5\implies q=5$
Else $p|(q-1)$
No comments:
Post a Comment