# Primitive root

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In number theory, a primitive root of a modulus is a number whose powers run through all the residue classes coprime to the modulus. This may be expressed by saying that n has a primitive root if the multiplicative group modulo n is cyclic, and the primitive root is a generator, having an order equal to Euler's totient function φ(n). Another way of saying that n has a primitive root is that the value of Carmichael's lambda function, λ(n) is equal to φ(n).

The numbers which possess a primitive root are:

• 2 and 4;
• ${\displaystyle p^{n}}$ and ${\displaystyle 2p^{n}}$ where p is an odd prime.

If g is a primitive root modulo an odd prime p, then one of g and g+p is a primitive root modulo ${\displaystyle p^{2}}$ and indeed modulo ${\displaystyle p^{n}}$ for all n.

There is no known fast method for determining a primitive root modulo p, if one exists. It is known that the smallest primitive root modulo p is of the order of ${\displaystyle p^{4/{\sqrt {e}}}}$ by a result of Burgess[1], and if the generalised Riemann hypothesis is true, this can be improved to an upper bound of ${\displaystyle 2(\log p)^{2}}$ by a result of Bach[2].

## Artin's conjecture

Artin's conjecture for primitive roots states that any number g which is not a perfect square is infinitely often a primitive root. Roger Heath-Brown has shown that there are at most two exceptional prime numbers a for which Artin's conjecture fails.[3]