Sharp Constants of Approximation Theory. VI. Weighted Polynomial Inequalities of Different Metrics on the Multidimensional Cube and Ball

Mapping Borel sets onto balls and self-similar sets by Lipschitz and nearly Lipschitz maps

If $X$ is an analytic metric space satisfying a very mild doubling condition, then for any finite Borel measure $μ$ on $X$ there is a set $N\subseteq X$ such that $μ(N)>0$, an ultrametric space $Z$ and a Lipschitz bijection $ϕ:N\to Z$ whose inverse is nearly Lipschitz, i.e., $β$-Hölder for all $β<1$. As an application it is shown that a Borel set in a Euclidean space maps onto $[0,1]^n$ by a nearly Lipschitz map if and only if it cannot be covered by countably many sets of Hausdorff dimension strictly below $n$. The argument extends to analytic metric spaces satisfying the mild condition. Furt…

带局部权重的庞加莱-索伯列夫不等式的推广

Generalization for Poincaré--Sobolev inequalities with local weights

本文研究了不假设Muckenhoupt条件时，依赖区域立方体的局部权重下庞加莱-索伯列夫不等式的加权推广，并将其应用于齐次与非齐次型索伯列夫嵌入定理的加权推广。

On the Small Ball Inequality in Three Dimensions

We prove an inequality related to questions in Approximation Theory, Probability Theory, and to Irregularities of Distribution. Let $h_R$ denote an $L ^{\infty}$ normalized Haar function adapted to a dyadic rectangle $R\subset [0,1] ^{3}$. We show that there is a postive $η$ so that for all integers $n$, and coefficients $ α(R)$ we have 2 ^{-n} \sum_{\abs{R}=2 ^{-n}} \abs{α(R)} {}\lesssim{} n ^{1 - η} \NOrm \sum_{\abs{R}=2 ^{-n}} α(R) h_R >.\infty . This is an improvement over the `trivial' estimate by an amount of $n ^{- η}$, and the optimal value of $η$ (which we do not prove) would be $ η=\…

Estimates for Littlewood--Paley Operators on Ball Campanato-Type Function Spaces

Let $X$ be a ball quasi-Banach function space on ${\mathbb R}^n$ and assume that the Hardy--Littlewood maximal operator satisfies the Fefferman--Stein vector-valued maximal inequality on $X$, and let $q\in[1,\infty)$ and $d\in(0,\infty)$. In this article, the authors prove that, for any $f\in \mathcal{L}_{X,q,0,d}(\mathbb{R}^n)$ (the ball Campanato-type function space associated with $X$), the Littlewood--Paley $g$-function $g(f)$ is either infinite everywhere or finite almost everywhere and, in the latter case, $g(f)$ is bounded on $\mathcal{L}_{X,q,0,d}(\mathbb{R}^n)$. Similar results for bo…

Hardy Spaces Associated with Ball Quasi-Banach Function Spaces on Spaces of Homogeneous Type: Characterizations of Maximal Functions, Decompositions, and Dual Spaces

Let $({\mathcal X},ρ,μ)$ be a space of homogeneous type in the sense of Coifman and Weiss, and $Y({\mathcal X})$ a ball quasi-Banach function space on ${\mathcal X}$, which supports a Fefferman--Stein vector-valued maximal inequality, and the boundedness of the powered Hardy--Littlewood maximal operator on its associate space. The authors first introduce the Hardy space $H_{Y}^*({\mathcal X})$, associated with $Y({\mathcal X})$, via the grand maximal function, and then establish its various real-variable characterizations, respectively, in terms of radial or non-tangential maximal functions, a…