Title: On congruence classes of orders of reductions of elliptic curves

Abstract: Let $E$ be an elliptic curve defined over $\mathbb{Q}$ and $\widetilde{E}_p(\mathbb{F}_p)$ denote the reduction of $E$ modulo a prime $p$ of good reduction for $E$. Given an integer $m\ge 2$ and any $a$ modulo $m$, we consider how often the congruence $|\widetilde{E}_p(\mathbb{F}_p)|\equiv a\mod m$ holds. We show that the greatest common divisor of the integers $|\widetilde{E}_p(\mathbb{F}_p)|$ over all rational primes $p$ cannot exceed $4$. We then exhibit elliptic curves over $\mathbb{Q}(t)$ with trivial torsion for which the orders of reductions of every smooth fiber modulo primes of positive density at least $1/2$ are divisible by a fixed small integer. We also show that if the torsion of $E$ grows over a quadratic field $K$, then one may explicitly compute $|\widetilde{E}_p(\mathbb{F}_p)|$ modulo the torsion subgroup $|E(K)_{tors}|$. More precisely, we show that there exists an integer $N\ge 2$ such that $|\widetilde{E}_p(\mathbb{F}_p)|$ is determined modulo $|E(K)_{tors}|$ according to the arithmetic progression modulo $N$ in which $p$ lies. It follows that given any $a$ modulo $|E(K)_{tors}|$, we can estimate the density of primes $p$ such that the congruence $|\widetilde{E}_p(\mathbb{F}_p)|\equiv a\mod |E(K)_{tors}|$ occurs.