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

Abstract

Let $Ω\in L^1{({\mathbb S^{n-1}})}$, be a function of homogeneous of degree zero, and $M_Ω$ be the Hardy-Littlewood maximal operator associated with $Ω$ defined by $M_Ω(f)(x) = \sup_{r>0}\frac1{r^n}\int_{|x-y| λ\}| = n^{-1}\|Ω\|_{L^1({\mathbb S^{n-1}})}\|f\|_{L^1({\mathbb R^n})}.$$ This removes the smoothness restrictions on the kernel $Ω$, such as Dini-type conditions, in previous results. To prove our result, we present a new upper bound of $\|M_Ω\|_{L^1\to L^{1,\infty}}$, which essentially improves the upper bound $C(\|Ω\|_{L\log L({\mathbb S^{n-1}})}+1)$ given by Christ and Rubio de Francia. As a consequence, the upper and lower bounds of $\|M_Ω\|_{L^1\to L^{1,\infty}}$ are obtained for $Ω\in L\log L {({\mathbb S^{n-1}})}$.