Arithmetic complexity revisited

Abstract

The arithmetic complexity counts the number of algebraically independent entries in the periodic continued fraction $\theta=[b_1,\dots, b_N, \overline{a_1,\dots,a_k}]$. If $\mathscr{A}_{\theta}$ is a noncommutative torus corresponding to the rational elliptic curve $\mathscr{E}(K)$, then the rank of $\mathscr{E}(K)$ is given by a simple formula $r(\mathscr{E}(K))= c(\mathscr{A}_{\theta})-1$, where $c(\mathscr{A}_{\theta})$ is the arithmetic complexity of $\theta$. We prove that $c(\mathscr{A}_{\theta})$ is equal to the dimension of the Brock-Elkies-Jordan variety of $\theta$ introduced in [1]. Following Zagier and Lemmermeyer, we evaluate the Shafarevich-Tate group of $\mathscr{E}(K)$.