The strong type endpoint estimate for the local spherical maximal function

Abstract

Let $p_d=(d^2+1)/(d(d-1))$ and $q_d=(d^2+1)/(d-1)$. We prove that the local spherical maximal operator $\sup_{1\leq t\leq 2}|f*dσ_t|$ is bounded from $L^{p_d}(\mathbb{R}^d)$ to $L^{q_d}(\mathbb{R}^d)$ for every $d\geq 2$. Previously only the restricted weak type estimate was known at this endpoint. Together with known results, this completes the characterization of the strong type $L^p\to L^q$ bounds for the local spherical maximal operator. We also prove the maximal estimate \[ \Big\|\sup_{t\in\mathbb{R}}|e^{it\sqrt{-Δ}}f|\Big\|_{L^{2(d+1)/(d-1)}(\mathbb{R}^d)} \lesssim\|f\|_{\dot H^{d/(d+1)}(\mathbb{R}^d)} \] for the wave equation at the critical exponent. Both results are obtained by exploiting the gain in bilinear estimates that comes from the interaction between high and low frequencies.