A Lang-Trotter Problem for Non-Geometric Quadratic Inductions

Source abstract

Let $K/\mathbb Q$ be an imaginary quadratic extension and $p$ an odd prime. Write $ρ=\operatorname{Ind}_{G_K}^{G_{\mathbb Q}}χ$, where $E/\mathbb Q_p$ is a finite extension and $χ:G_K\to\mathcal O_E^\times$ is a continuous character. For a fixed $r\in\mathbb Z\setminus\{0\}$, let $π_{ρ,r}(X)$ denote the number of rational primes $\ell\le X$ such that $ρ$ is unramified at $\ell$ and $\operatorname{tr}ρ(\operatorname{Frob}_\ell)=r$. Let $a,b$ be the two weights of $χ$ at $p$. We prove that if $(a,b)\notin\mathbb Q^2$, then $π_{ρ,r}(X)\ll_{ρ,r,\varepsilon}X^\varepsilon$ for every $\varepsilon>0$, while if $(a,b)\in\mathbb Q^2\setminus\mathbb Z^2$, then only finitely many such primes occur. These bounds are substantially sparser than the classical CM Lang--Trotter scale. The main input in the non-rational case is a rigidity theorem for algebraic curves in the $p$-adic analytic trace locus, combined with rigid-analytic Pila--Wilkie counting; the rational non-integral case is treated by a local ramification argument.