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Commentarii Mathematici Helvetici

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A prime analogue of the Erdös--Pomerance conjecture for elliptic curves

Yu-Ru Liu (1)

(1) Department of Pure Mathematics, University of Waterloo, N2L 3G1, WATERLOO, ONTARIO, CANADA

Let $E/{\mathbb Q}$ be an elliptic curve of rank $\ge 1$ and $b\in E({\mathbb Q})$ a rational point of infinite order. For a prime $p$ of good reduction, let $g_b(p)$ be the order of the cyclic group generated by the reduction $\bar b$ of $b$ modulo $p$. We denote by $\omega(g_b(p))$ the number of distinct prime divisors of $g_b(p)$. Assuming the GRH, we show that the normal order of $\omega(g_b(p))$ is $\log \log p$. We also prove conditionally that there exists a normal distribution for the quantity $$ \frac{\omega(g_b(p)) - \log \log p}{\sqrt{\log \log p}}. $$ The latter result can be viewed as an elliptic analogue of a conjecture of Erdös and Pomerance about the distribution of $\omega(f_a(n))$, where $a$ is a natural number $> 1$ and $f_a(n)$ the order of $a$ modulo $n$.

Keywords: Prime divisors, order of cyclic groups, elliptic curves