# Finite field with $9$th primitive root of unity.

I like to find those finite field $GF(p^n)$ which contains primitive $9$th root of unity. One thing is clear that $GF(p^n)^*$ is cyclic group of size $p^n-1.$ So for $9$th primitive root we must have $9/(p^n-1).$ i.e. $$p^n-1\equiv 0\mod9$$ so for primes of the form $p=9n+1$ finite field $GF(p^n)$ must have $9$th primitive root of unity. Please help me to find all prime $p$ so that $GF(p^n)$ has $9$th primitive root of unity. Thanks.

• Clearly you need $p\neq3$. Whether $GF(p^n)$ has a ninth primitive root will depend on the residue class of $p$ modulo $9$ and on the residue class of $n$ modulo $\phi(9)=6$. See the answer by lhf for the details. – Jyrki Lahtonen Mar 5 '18 at 21:00

A $9$th primitive root of unity is an element of order $9$.

In a cyclic group of order $m$, there is an element of order $d$ iff $d$ divides $m$.

Therefore, as you have found, $GF(p^n)^\times$ has an element of order $9$ iff $p^n\equiv 1\bmod 9$.

Since $p$ cannot be $3$, we have $\gcd(p,9)=1$ and so $p^6\equiv 1 \bmod 9$ and $1 \equiv p^n\equiv p^{n \bmod 6}\bmod 9$.

If you're only interested in $p$ but not in $n$, then the answer is: all $p\ne 3$.

If you're interested in which $n$ work for which $p$, then:

• $n \equiv 1 \bmod 6$ implies $p \equiv 1 \bmod 9$

• $n \equiv 2 \bmod 6$ implies $p^2 \equiv 1 \bmod 9$, that is, $p \equiv \pm1 \bmod 18$

• $n \equiv 3 \bmod 6$ implies $p^3 \equiv 1 \bmod 9$, that is, $p \equiv 1 \bmod 6$

• $n \equiv 6 \bmod 6$ implies $p^6 \equiv 1 \bmod 9$, that is, $p \equiv \pm1 \bmod 3$

The cases $n \equiv 4,5 \bmod 6$ reduce to $n \equiv 2,1 \bmod 6$ because $p^6\equiv 1\equiv p^n\bmod 9$ implies $p^{\gcd(n,6)} \equiv 1\bmod 9$.

• You are saying all $p\ne 3$ will work but how $GF(11)$ will work – neelkanth Mar 6 '18 at 3:27
• @neelkanth, for $p=11$ you need to take $n \equiv 6 \bmod 6$. – lhf Mar 6 '18 at 11:02
• What lhf is saying is, all $p,n$ such that 9 divides $p^n-1$. – Gerry Myerson Mar 6 '18 at 12:09
• Ok I misunderstood it....... – neelkanth Mar 6 '18 at 13:02
• @Ihf what if $n\equiv 4 \mod 6$? or $n\equiv 5 \mod 6$? – neelkanth Mar 13 '18 at 10:36