Refinements of strong multiplicity one for $\mathrm{GL}(2)$

Abstract

For distinct unitary cuspidal automorphic representations $\pi_1$ and $\pi_2$ for $\mathrm{GL}(2)$ over a number field $F$ and any $\alpha\in\Bbb{R}$, let $\mathcal{S}_{\alpha}$ be the set of primes $v$ of $F$ for which $\lambda_{\pi_1}(v)\neq e^{i\alpha} \lambda_{\pi_2}(v)$, where $\lambda_{\pi_i}(v)$ is the Fourier coefficient of $\pi_i$ at $v$. In this article, we show that the lower Dirichlet density of $\mathcal{S}_\alpha$ is at least $\frac{1}{16}$. Moreover, if $\pi_1$ and $\pi_2$ are not twist-equivalent, we show that the lower Dirichlet densities of $\mathcal{S}_\alpha$ and $ \cap_\alpha\mathcal{S}_\alpha$ are at least $\frac{2}{13}$ and $\frac{1}{11}$, respectively. Furthermore, for non-twist-equivalent $\pi_1$ and $\pi_2$, if each $\pi_i$ corresponds to a non-CM newform of weight $k_i\ge 2$ and with trivial nebentypus, we obtain various upper bounds for the number of primes $p\le x$ such that $\lambda_{\pi_1}(p)^2 = \lambda_{\pi_2}(p)^2$. These present refinements of the works of Murty-Pujahari, Murty-Rajan, Ramakrishnan, and Walji.