Gradient bounds for radial maximal functions

Emanuel Carneiro¹ ; Cristian González-Riquelme²

¹ICTP - The Abdus Salam International Centre for Theoretical Physics and IMPA - Instituto de Matemática Pura e Aplicada
²IMPA - Instituto de Matemática Pura e Aplicada

Annales Fennici Mathematici, Tome 46 (2021) no. 1, pp. 495-521

Résumé

In this paper we study the regularity properties of certain maximal operators of convolution type at the endpoint $p=1$, when acting on radial data. In particular, for the heat flow maximal operator and the Poisson maximal operator, when the initial datum $u_0 \in W^{1,1}(\mathbf{R}^d)$ is a radial function, we show that the associated maximal function $u^*$ is weakly differentiable and \[\|\nabla u^*\|_{L^1(\mathbf{R}^d)} \lesssim_d \|\nabla u_0\|_{L^1(\mathbf{R}^d)}.\] This establishes the analogue of a recent result of Luiro for the uncentered Hardy-Littlewood maximal operator, now in a centered setting with smooth kernels. In a second part of the paper, we establish similar gradient bounds for maximal operators on the sphere $\mathbf{S}^d$, when acting on polar functions. Our study includes the uncentered Hardy-Littlewood maximal operator, the heat flow maximal operator and the Poisson maximal operator on $\mathbf{S}^d$.

Détail

Keywords: Maximal operators, Sobolev spaces, bounded variation, convolution, sphere