Theta functions, fourth moments of eigenforms, and the sup-norm problem II (2207.12351v1)

Abstract: For an $L^2$-normalized holomorphic newform $f$ of weight $k$ on a hyperbolic surface of volume $V$ attached to an Eichler order of squarefree level in an indefinite quaternion algebra over $\mathbb{Q}$, we prove the sup-norm estimate [ | \Im(\cdot)^{\frac{k}{2}} f |{\infty} \ll{\epsilon} (k V)^{\frac{1}{4}+\epsilon} ] with absolute implied constant. For a cuspidal Maa{\ss} newform $\varphi$ of eigenvalue $\lambda$ on such a surface, we prove that [ |\varphi |{\infty} \ll{\lambda,\epsilon} V^{\frac{1}{4}+\epsilon}. ] We establish analogous estimates in the setting of definite quaternion algebras.