# Prove that $r$ is a primitive root modulo $p$ if and only if $r^{(p−1)/q}\not\equiv 1\pmod{p}$

Suppose $p$ is an odd prime. Prove that $r$, with $\gcd(r, p) = 1$, is a primitive root modulo $p$ if and only if $r^{(p−1)/q}\not\equiv 1\pmod{p}$ for all prime divisors $q$ of $p − 1$.

The only if direction is trivial, I can multiply $q$ to get $(r^{(p-1)/q})^q\equiv1^q\pmod{p}$. So, $r$ is a primitive root. How do I do the other direction? Can someone give me a hint or suggestion? Thanks

## 2 Answers

Hints. The primitive roots mod $p$ are exactly the generators of $(\mathbb{Z}/p\mathbb{Z})^\times\cong\mathbb{Z}/(p-1)\mathbb{Z}$. Hence, $r$ is a primitive root mod $p$ if and only if the order of $r$ is $p-1$. Besides, since $r\in(\mathbb{Z}/p\mathbb{Z})^\times$, using Lagrange's theorem the order of $r$ is a divisor of $p-1$.

By definition: $\ a$ is a primitive root $\iff a\,$ has order $\,p\!-\!1.\$ Now apply the fundamental

Order Test $\ \,a\,$ has order $\,n\iff a^n \equiv 1\,$ but $\,\color{#0a0}{a^{n/q} \not\equiv 1}\,$ for every prime $\,q\mid n$

Proof $\,\ (\Leftarrow)\,\$ Let $\,a\,$ have $\,\color{#c00}{{\rm order}\ k}.\,$ Then $\,k\mid n\,$ (proof).  If $\:k < n\,$ then $\,k\,$ is proper divisor of $\,n\,$ therefore $\,k\,$ arises by deleting at least one prime $\,q\,$ from the prime factorization of $\,n,\,$ hence $\,k\mid n/q,\,$ say $\, kj = n/q,\$ so $\ \color{#0a0}{a^{\large n/q}} \equiv (\color{#c00}{a^{\large k}})^{\large j}\equiv \color{#c00}1^{\large j}\equiv\color{#0a0}1\,$ contra $\rm\color{#0a0}{hypothesis}$. So $\,k=n.$ $\ (\Rightarrow)\$ Clear.

• Remark  The claim about divisibility by deletion of prime factors is an immediate consequence of existence and uniqueness of prime factorizations. – Bill Dubuque Nov 8 '16 at 0:11