# Artin's conjecture on primitive roots

In number theory, Artin's conjecture on primitive roots states that a given integer a that is neither a square number nor −1 is a primitive root modulo infinitely many primes p. The conjecture also ascribes an asymptotic density to these primes. This conjectural density equals Artin's constant or a rational multiple thereof.
The conjecture was made by Emil Artin to Helmut Hasse on September 27, 1927, according to the latter's diary. The conjecture is still unresolved as of 2024. In fact, there is no single value of a for which Artin's conjecture is proved.

## Formulation

Let a be an integer that is not a square number and not −1. Write a = a$_0$b$^2$ with a$_0$ square-free. Denote by S(a) the set of prime numbers p such that a is a primitive root modulo p. Then the conjecture states
* S(a) has a positive asymptotic density inside the set of primes. In particular, S(a) is infinite.
* Under the conditions that a is not a perfect power and a$_0$ is not congruent to 1 modulo 4 (sequence A085397 in the OEIS), this density is independent of a and equals Artin's constant, which can be expressed as an infinite product
    $C_{\mathrm {Artin} }=\prod _{p\ \mathrm {prime} }\left(1-{\frac {1}{p(p-1)}}\right)=0.3739558136\ldots$ (sequence A005596 in the OEIS).
The positive integers satisfying these conditions are:
    2, 3, 6, 7, 10, 11, 12, 14, 15, 18, 19, 22, 23, 24, 26, 28, 30, 31, 34, 35, 38, 39, 40, 42, 43, 44, 46, 47, 48, 50, 51, 54, 55, 56, 58, 59, 60, 62, 63, … (sequence A085397 in the OEIS)
The negative integers satisfying these conditions are:
    2, 4, 5, 6, 9, 10, 13, 14, 16, 17, 18, 20, 21, 22, 24, 25, 26, 29, 30, 33, 34, 36, 37, 38, 40, 41, 42, 45, 46, 49, 50, 52, 53, 54, 56, 57, 58, 61, 62, … (sequence A120629 in the OEIS)
Similar conjectural product formulas exist for the density when a does not satisfy the above conditions. In these cases, the conjectural density is always a rational multiple of C$_{Artin}$. If a is a square number or a = −1, then the density is 0, and if a is a perfect pth power for prime p, then the number needs to be multiplied by ${\frac {p(p-2)}{p^{2}-p-1}}$ (if there are more than one such prime p, then the number needs to be multiplied by ${\frac {p(p-2)}{p^{2}-p-1}}$ for all such primes p), and if a$_0$ is congruent to 1 mod 4, then the number needs to be multiplied by ${\frac {p(p-1)}{p^{2}-p-1}}$ for all prime factors p of a$_0$, e.g. for a = 8 = 2$^3$, the density is ${\frac {3}{5}}C$, and for a = 5 (which is congruent to 1 mod 4), the density is ${\frac {20}{19}}C$.

## Example

For example, take a = 2. The conjecture claims that the set of primes p for which 2 is a primitive root has the above density C$_{Artin}$. The set of such primes is (sequence A001122 in the OEIS)
    S(2) = {3, 5, 11, 13, 19, 29, 37, 53, 59, 61, 67, 83, 101, 107, 131, 139, 149, 163, 173, 179, 181, 197, 211, 227, 269, 293, 317, 347, 349, 373, 379, 389, 419, 421, 443, 461, 467, 491, ...}.
It has 38 elements smaller than 500 and there are 95 primes smaller than 500. The ratio (which conjecturally tends to C$_{Artin}$) is 38/95 = 2/5 = 0.4.

## Partial results

In 1967, Christopher Hooley published a conditional proof for the conjecture, assuming certain cases of the generalized Riemann hypothesis.
Without the generalized Riemann hypothesis, there is no single value of a for which Artin's conjecture is proved. D. R. Heath-Brown proved in 1986 (Corollary 1) that at least one of 2, 3, or 5 is a primitive root modulo infinitely many primes p. He also proved (Corollary 2) that there are at most two primes for which Artin's conjecture fails.

## Some variations of Artin's problem
### Elliptic curve

An elliptic curve $E$ given by $y^{2}=x^{3}+ax+b$, Lang and Trotter gave a conjecture for rational points on $E(\mathbb {Q} )$ analogous to Artin's primitive root conjecture.
Specifically, they said there exists a constant $C_{E}$ for a given point of infinite order $P$ in the set of rational points $E(\mathbb {Q} )$ such that the number $N(P)$ of primes ($p\leq x$) for which the reduction of the point $P{\pmod {p}}$ denoted by ${\bar {P}}$ generates the whole set of points in $\mathbb {F_{p}}$ in $E$, denoted by ${\bar {E}}(\mathbb {F_{p}} )$, is given by $N(P)\sim C_{E}\left({\frac {x}{\log x}}\right)$. Here we exclude the primes which divide the denominators of the coordinates of $P$.
Gupta and Murty proved the Lang and Trotter conjecture for $E/\mathbb {Q}$ with complex multiplication under the Generalized Riemann Hypothesis, for primes splitting in the relevant imaginary quadratic field.

### Even order

Krishnamurty proposed the question how often the period of the decimal expansion $1/p$ of a prime $p$ is even.
The claim is that the period of the decimal expansion of a prime in base $g$ is even if and only if $g^{\left({\frac {p-1}{2^{j}}}\right)}\not \equiv 1{\bmod {p}}$ where $j\geq 1$ and $j$ is unique and p is such that $p\equiv 1+2^{j}\mod {2^{j}}$.
The result was proven by Hasse in 1966.