On the Bounds of Weak $(1,1)$ Norm of Hardy-Littlewood Maximal Operator with $L\log L({\mathbb S^{n-1}})$ Kernels

TL;DR

This paper establishes the limiting weak-type behavior of the Hardy-Littlewood maximal operator with kernels in L log L on the sphere, removing smoothness restrictions and improving bounds on the operator's weak-type norm.

Contribution

It proves the limiting weak-type behavior for kernels in L log L, removing previous smoothness restrictions, and provides new bounds on the operator's weak-type norm.

Findings

Limit of λ times measure of level set as λ approaches 0 is proportional to the L1 norm of the kernel and the function.

New upper bounds for the L1 to weak-L1 operator norm are established, improving previous results.

Exact bounds of the operator norm are obtained for kernels in L log L.

Abstract

Let $\Omega\in L^1{({\mathbb S^{n-1}})}$, be a function of homogeneous of degree zero, and $M_\Omega$ be the Hardy-Littlewood maximal operator associated with $\Omega$ defined by $M_\Omega(f)(x) = \sup_{r>0}\frac1{r^n}\int_{|x-y|<r}|\Omega(x-y)f(y)|dy.$ It was shown by Christ and Rubio de Francia that $\|M_\Omega(f)\|_{L^{1,\infty}({\mathbb R^n})} \le C(\|\Omega\|_{L\log L({\mathbb S^{n-1}})}+1)\|f\|_{L^1({\mathbb R^n})}$ provided $\Omega\in L\log L {({\mathbb S^{n-1}})}$. In this paper, we show that, if $\Omega\in L\log L({\mathbb S^{n-1}})$, then for all $f\in L^1({\mathbb R^n})$, $M_\Omega$ enjoys the limiting weak-type behaviors that $$\lim_{\lambda\to 0^+}\lambda|\{x\in{\mathbb R^n}:M_\Omega(f)(x)>\lambda\}| = n^{-1}\|\Omega\|_{L^1({\mathbb S^{n-1}})}\|f\|_{L^1({\mathbb R^n})}.$$ This removes the smoothness restrictions on the kernel $\Omega$, such as Dini-type conditions, in previous results. To prove our result, we present a new upper bound of $\|M_\Omega\|_{L^1\to L^{1,\infty}}$, which essentially improves the upper bound $C(\|\Omega\|_{L\log L({\mathbb S^{n-1}})}+1)$ given by Christ and Rubio de Francia. As a consequence, the upper and lower bounds of $\|M_\Omega\|_{L^1\to L^{1,\infty}}$ are obtained for $\Omega\in L\log L {({\mathbb S^{n-1}})}$.