# Primitive root

• Nov 19th 2013, 04:47 PM
Sonifa
Primitive root
p prime, If p=1 ( mod 3) then Zp contains primitive cube
roots of unity. Now I am considering which p does Zp contains primitive fourth
roots of unity.

I can prove that if Zp contains primitive fourth roots of
unity, then 4|(p-1) . but how about the opposite way? I mean if p=1(mod4) then
Zp contains primitive fourth roots of unity?? I know this statements true if q
prime instead of 4. And what values of p does Zp contains primitive fourth roots
of unity???
• Nov 19th 2013, 05:41 PM
johng
Re: Primitive root
Hi,
By a primitive 4th root of unity, I assume you mean a y such that y4 is 1 mod p, but no smaller power is 1. Now if you know about cyclic groups, your problem is easy. However, even without this here's a proof.

There is a primitive 4th root of unity mod p if and only if 4 divides p-1.
Proof of only if:
Let x be a primitive root mod p; i.e. every non-zero element of Zp is a power of x. (You need to know such exist.) So suppose y=xm is a primitive 4th root. I need the following fact: the order of xm mod p is (p-1)/gcd(m,p-1). (Order is the least positive power k with (xm)k = 1 mod p; this is a standard result from cyclic groups. But it's not hard to prove separately.) So p-1=4gcd(m,p-1) or 4 divides p-1. QED.