Congruences of elliptic curves arising from non-surjective mod $N$ Galois representations

Abstract: We study $N$-congruences between quadratic twists of elliptic curves. If $N$ has exactly two distinct prime factors we show that these are parametrised by double covers of certain modular curves. In many, but not all cases, the modular curves in question correspond to the normaliser of a Cartan subgroup of $\mathrm{GL}_2(\mathbb{Z}/N\mathbb{Z})$. By computing explicit models for these double covers we find all pairs $(N, r)$ such that there exist infinitely many $j$-invariants of elliptic curves $E/\mathbb{Q}$ which are $N$-congruent with power $r$ to a quadratic twist of $E$. We also find an example of a $48$-congruence over $\mathbb{Q}$. We make a conjecture classifying nontrivial $(N,r)$-congruences between quadratic twists of elliptic curves over $\mathbb{Q}$. Finally, we give a more detailed analysis of the level $15$ case. We use elliptic Chabauty to determine the rational points on a modular curve of genus $2$ whose Jacobian has rank $2$ and which arises as a double cover of the modular curve $X(\mathrm{ns} 3^+, \mathrm{ns} 5^+)$. As a consequence we obtain a new proof of the class number $1$ problem.