Almost prime values of the order of elliptic curves over finite fields

Abstract. Let E $E$ be an elliptic curve over ${\mathbb {Q}}$ without complex multiplication. For each prime p $p$ of good reduction, let | E ( p ) | $|E({\mathbb {F}}_p)|$ be the order of the group of points of the reduced curve over p ${\mathbb {F}}_p$ . According to a conjecture of Koblitz, there should be infinitely many such primes p $p$ such that | E ( p ) | $|E({\mathbb {F}}_p)|$ is prime, unless there are some local obstructions predicted by the conjecture. Suppose that E $E$ is a curve without local obstructions (which is the case for most elliptic curves over ${\mathbb {Q}}$ ). We prove in this paper that, under the GRH, there are at least 2 . 778 C E twin x / ( log x ) 2 $2.778 C_E^{\rm twin} x / (\log x)^2$ primes p $p$ such that | E ( p ) | $|E({\mathbb {F}}_p)|$ has at most 8 prime factors, counted with multiplicity. This improves previous results of Steuding & Weng [20, 21] and Miri & Murty [15]. This is also the first result where the dependence on the conjectural constant C E twin $C_E^{\rm twin}$ appearing in Koblitz's conjecture (also called the twin prime conjecture for elliptic curves) is made explicit. This is achieved by sieving a slightly different sequence than the one of [20] and [15]. By sieving the same sequence and using Selberg's linear sieve, we can also improve the constant of Zywina [24] appearing in the upper bound for the number of primes p $p$ such that | E ( p ) | $|E({\mathbb {F}}_p)|$ is prime. Finally, we remark that our results still hold under an hypothesis weaker than the GRH.