3 is a primitive root modulo primes of the form 16 q^4 + 1, where q>3 is prime

Importance: Medium ✭✭
Subject: Number Theory
Recomm. for undergrads: no
Posted by: princeps
on: February 25th, 2012
Conjecture   $ 3~ $ is a primitive root modulo $ ~p $ for all primes $ ~p=16\cdot q^4+1 $, where $ ~q>3 $ is prime.


* indicates original appearance(s) of problem.

group theory answer

Using group theory, the multiplicative group of order p=16q^4+1 has order p-1=16q^4. Using lagrange's theorem, the order of any element divides the order of the group. Therefore, any element is either a primitive root, a quadratic residue, or a qth power residue mod 16q^4+1. Using the laws of quadratic reciprocity, 3 is a quadratic residue modulo a prime if and only if the prime is congruent to plus or minus 1 mod 12. Since q>3 is a prime and therefore not divisible by 3, 16q^4=1(mod 3), so 16q^4+1=2(mod 3). That means that 16q^4+1=5(mod 12), and therefore 3 is not a quadratic residue mod p. Therefore the only thing left to prove is that 3 is not a qth power residue.

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