Published online by Cambridge University Press: 20 November 2018
Let $E$ be an elliptic curve over $\mathbb{Q}$ , and let $r$ be an integer. According to the Lang-Trotter conjecture, the number of primes $p$ such that ${{a}_{p}}\left( E \right)=r$ is either finite, or is asymptotic to ${{C}_{E,r}}\sqrt{x}/\log x$ where ${{C}_{E,r}}$ is a non-zero constant. A typical example of the former is the case of rational $\ell $-torsion, where ${{a}_{p}}\left( E \right)=r$ is impossible if $r\equiv 1\,\left( \bmod \,\ell \right)$. We prove in this paper that, when $E$ has a rational $\ell $-isogeny and $\ell \ne 11$, the number of primes $p$ such that ${{a}_{p}}\left( E \right)\equiv r\,\left( \bmod \,\ell \right)$ is finite (for some $r$ modulo $\ell $) if and only if $E$ has rational $\ell $-torsion over the cyclotomic field $\mathbb{Q}\left( {{\zeta }_{\ell }} \right)$ . The case $\ell =11$ is special, and is also treated in the paper. We also classify all those occurences.