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Dachun Yang

Publications and source records attributed to Dachun Yang.

At least 55 records · Page 3Linked to original sources

arXiv2025

Robin Problems of Elliptic Equations on Rough Domains: Hölder Regularity, Green's Functions, and Harmonic Measures

Let $n\ge 2$ and $s\in (n-2,n)$. Assume that $Ω\subset \mathbb{R}^n$ is a one-sided bounded non-tangentially accessible domain with $s$-Ahlfors regular boundary and $σ$ is the surface measure on the boundary of $Ω$, denoted by $\partial Ω$. Let $β$ be a non-negative measurable function on $\partial Ω$ satisfying $β\in L^{q_0}(\partial Ω,σ)~\text{with}~ q_0 \in(\frac{s}{s+2-n},\infty]~\text{and}\ β\ge a_0~\text{on}~E_0\subset \partial Ω, $ where $a_0$ is a given positive constant and $E_0\subset \partial Ω$ is a $σ$-measurable set with $σ(E_0)>0$. In this article, for any $f\in L^p(\partial Ω,σ)$ with $p\in(s/(s+2-n),\infty]$, we obtain the existence and uniqueness, the global Hölder regularity, and the boundary Harnack inequality of the weak solution to the Robin problem $$\begin{cases} -\mathrm{div}(A\nabla u) = 0~~&\text{in}~Ω,\\ A\nabla u\cdot \boldsymbolν+βu = f~~&\text{on}~\partial Ω, \end{cases} $$ where the coefficient matrix $A$ is real-valued, bounded and measurable and satisfies the uniform ellipticity condition and where $\boldsymbolν$ denotes the outward unit normal to $\partialΩ$. Furthermore, we establish the existence, upper bound pointwise estimates, and the Hölder regularity of Green's functions associated with this Robin problem. As applications, we further prove that the harmonic measure associated with this Robin problem is mutually absolutely continuous with respect to the surface measure $σ$ and also provide a quantitative characterization of mutual absolute continuity at small scales. These results extend the corresponding results established by David et al. [arXiv: 2410.23914] via weakening their assumption that $β$ is a given positive constant.

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arXiv2025

Fractional Heat Semigroup Characterization of Distances from Functions in Lipschitz Spaces to Their Subspaces

Let $Λ_s$ denote the inhomogeneous Lipschitz space of order $s\in(0,\infty)$ on $\mathbb{R}^n$. This article characterizes the distance $d(f, V)_{Λ_s}: = \inf_{g\in V} \|f-g\|_{Λ_s}$ from a function $f\in Λ_s$ to a non-dense subspace $V\subset Λ_s$ via the fractional semigroup $\{T_{α, t}: =e^{-t (-Δ)^{α/2}}: t\in (0, \infty)\}$ for any $α\in(0,\infty)$. Given an integer $ r >s/α$, a uniformly bounded continuous function $f$ on $\mathbb{R}^n$ belongs to the space $Λ_s$ if and only if there exists a constant $λ\in(0,\infty)$ such that \begin{align*} \left|(-Δ)^{\frac {αr}2} (T_{α, t^α} f)(x) \right|\leq λt^{s -rα}\ \ \text{for any $x\in\mathbb{R}^n$ and $t\in (0, 1]$}.\end{align*} The least such constant is denoted by $λ_{ α, r, s}(f)$. For each $f\in Λ_s$ and $0<\varepsilon< λ_{α,r, s}(f)$, let $$ D_{α, r}(s,f,\varepsilon):=\left\{ (x,t)\in \mathbb{R}^n\times (0,1]:\ \left| (-Δ)^{\frac {αr}2} (T_{α, t^α} f)(x) \right|> \varepsilon t^{s -r α}\right\}$$ be the set of ``bad'' points. To quantify its size, we introduce a class of extended nonnegative \emph{admissible set functions} $ν$ on the Borel $σ$-algebra $\mathcal{B}(\mathbb{R}^n\times [0, 1])$ and define, for any admissible function $ν$, the \emph{critical index} $ \varepsilon_{α, r, s,ν}(f):=\inf\{\varepsilon\in(0,\infty):\ ν(D_{α, r}(s,f,\varepsilon))<\infty\}.$ Our result shows that, for a broad class of subspaces $V\subset Λ_s$, including intersections of $Λ_s$ with Sobolev, Besov, Triebel--Lizorkin, and Besov-type spaces, there exists an admissible function $ν$ depending on $V$ such that $\varepsilon_{α, r, s,ν}(f)\sim \mathrm{dist}(f, V)_{Λ_s}.$

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arXiv2025

Matrix-Weighted Campanato Spaces: Duality and Calderón--Zygmund Operators

Let $p\in(0,\infty)$, $q\in[1,\infty)$, $s\in\mathbb Z_+$, and $W$ be an $A_p$-matrix weight, which in the scalar case is exactly a Muckenhoupt $A_{\max\{1,p\}}$ weight. In this article, by using the reducing operators of $W$, we introduce matrix-weighted Campanato spaces $\mathcal L_{p,q,s,W}$. When $p\in(0,1]$, applying the atomic and the finite atomic characterizations of the matrix-weighted Hardy space $H^p_W$, we prove that the dual space of $H^p_W$ is precisely $\mathcal L_{p,q,s,W}$, which further induces several equivalent characterizations of $\mathcal L_{p,q,s,W}$. In addition, we obtain a necessary and sufficient condition for the boundedness of modified Calderón--Zygmund operators on $\mathcal L_{p,q,s,W}$ with $p\in(0,\infty)$, which, combined with the duality, further gives a necessary and sufficient condition for the boundedness of Calderón--Zygmund operators on $H^p_W$ with $p\in(0,1]$.

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arXiv2025

Characterization of Vanishing Campanato Spaces via Ball Banach Function Spaces and Its Applications

In this article, the authors provide some new characterizations of several vanishing Campanato spaces using a type of oscillation defined within the general framework of ball Banach function spaces. This approach yields fresh insights even in the special case of the vanishing BMO space. The characterization reveals a self-improvement phenomenon inherent in vanishing Campanato spaces. A key innovation of this approach lies in using higher-order differences to dominate oscillations. Instead of directly estimating these differences, the authors achieve the domination by smoothing the function via convolution. As additional outcomes, the authors also obtain new characterizations of vanishing Campanato spaces in terms of higher-order differences. Finally, the authors present several examples to show that these vanishing Campanato spaces naturally arise in the study on the compactness of fractional integral commutators in Morrey spaces.

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arXiv2025

Sharp Weighted Cohen--Dahmen--Daubechies--DeVore Inequality with Applications to (Weighted) Critical Sobolev Spaces, Gagliardo--Nirenberg Inequalities, and Muckenhoupt Weights

In this article, we establish a quantitative weighted variant of a far-reaching inequality obtained by A. Cohen, W. Dahmen, I. Daubechies, and R. DeVore in 2003, whose dependence on the $A_p$-weight constant for any $p\in[1,\infty)$ is sharp. As applications, we obtain the almost characterization of the critical weighted Sobolev space in terms of wavelets, a sharp real interpolation between this weighted Sobolev space and weighted Besov spaces, and three new Gagliardo--Nirenberg type inequalities in the framework of ball Banach function spaces. Moreover, we apply this sharp weighted inequality to extend the famous Brezis--Seeger--Van Schaftingen--Yung formula in ball Banach function spaces, which gives an affirmative answer to the question in page 29 of [Calc. Var. Partial Differential Equations 62 (2023), Paper No. 234]. Notably, we further establish two new characterizations of Muckenhoupt weights related to the inequality of Cohen et al.\ and the formula of Brezis et al. The most novelty of this article exists in applying and further developing the good cube method introduced by Cohen et al.\ to trace the sharp dependences on weight constants.

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arXiv2025

Equivalent Characterizations and Applications of Fractional Sobolev Spaces with Partially Vanishing Traces on $(ε,δ,D)$-Domains Supporting $D$-Adapted Fractional Hardy Inequalities

Let $Ω\subset\mathbb{R}^n$ be an $(ε,δ,D)$-domain, with $ε\in(0,1]$, $δ\in(0,\infty]$, and $D\subset \partial Ω$ being a closed part of $\partial Ω$, which is a general open connected set when $D=\partial Ω$ and an $(ε,δ)$-domain when $D=\emptyset$. Let $s\in(0,1)$ and $p\in[1,\infty)$. If ${W}^{s,p}(Ω)$, ${\mathcal W}^{s,p}(Ω)$, and $\mathring{W}_D^{s,p}(Ω)$ are the fractional Sobolev spaces on $Ω$ that are defined respectively via the restriction of $W^{s,p}(\mathbb{R}^n)$ to $Ω$, the intrinsic Gagliardo norm, and the completion of all $C^\infty(Ω)$ functions with compact support away from $D$, in this article we prove their equivalences [that is, ${W}^{s,p}(Ω)={\mathcal{W}}^{s,p}(Ω) =\mathring{W}_D^{s,p}(Ω)$] if $Ω$ supports a $D$-adapted fractional Hardy inequality and, moreover, when $sp\ne 1$ such a fractional Hardy inequality is shown to be necessary to guarantee these equivalences under some mild geometric conditions on $Ω$. Using the aforementioned equivalences, we show that the real interpolation space $(L^p(Ω), \mathring{W}_D^{1,p}(Ω))_{s,p}$ equals to some weighted fractional order Sobolev space $\mathcal{W}^{s,p}_{d_D^s}(Ω)$ when $p\in (1,\infty)$. Applying this to the elliptic operator $\mathcal{L}_D$ in $Ω$ with mixed boundary condition, we characterize both the domain of its fractional power and the parabolic maximal regularity of its Cauchy initial problem by means of $\mathcal{W}^{s,p}_{d_D^s}(Ω)$.

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arXiv2025

Equivalent Characterizations and Their Applications of Solvability of $L^p$ Poisson--Robin(-Regularity) Problems on Rough Domains

Let $n\ge2$, $Ω\subset\mathbb{R}^n$ be a bounded one-sided chord arc domain, and $p\in(1,\infty)$. In this article, we study the (weak) $L^p$ Poisson--Robin(-regularity) problem for a uniformly elliptic operator $L:=-\mathrm{div}(A\nabla\cdot)$ of divergence form on $Ω$, which considers weak solutions to the equation $Lu=h-\mathrm{div}\boldsymbol{F}$ in $Ω$ with the Robin boundary condition $A\nabla u\cdot\boldsymbolν+αu=\boldsymbol{F}\cdot\boldsymbolν$ on the boundary $\partialΩ$ for functions $h$ and $\boldsymbol{F}$ in some tent spaces. Precisely, we establish several equivalent characterizations of the solvability of the (weak) $L^p$ Poisson--Robin(-regularity) problem and clarify the relationship between the $L^p$ Poisson--Robin(-regularity) problem and the classical $L^p$ Robin problem. Moreover, we also give an extrapolation property for the solvability of the classical $L^p$ Robin problem. As applications, we further prove that, for the Laplace operator $-Δ$ on the bounded Lipschitz domain $Ω$, the $L^p$ Poisson--Robin and the $L^q$ Poisson--Robin-regularity problems are respectively solvable for $p\in(2-\varepsilon_1,\infty)$ and $q\in(1,2+\varepsilon_2)$, where $\varepsilon_1\in(0,1]$ and $\varepsilon_2\in(0,\infty)$ are constants depending only on $n$ and the Lipschitz constant of $Ω$ and, moreover, these ranges $(2-\varepsilon_1,\infty)$ of $p$ and $(1,2+\varepsilon_2)$ of $q$ are sharp. The main results in this article are the analogues of the corresponding results of both the $L^p$ Poisson--Dirichlet(-regularity) problem, established by M. Mourgoglou, B. Poggi and X. Tolsa [J. Eur. Math. Soc. 2025], and of the $L^p$ Poisson--Neumann(-regularity) problem, established by J. Feneuil and L. Li [arXiv: 2406.16735], in the Robin case.

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arXiv2025

Sharp Brezis--Seeger--Van Schaftingen--Yung Formulae for Higher-Order Gradients in Ball Banach Function Spaces

Let $X$ be a ball Banach function space on $\mathbb{R}^n$, $k\in\mathbb{N}$, $h\in\mathbb{R}^n$, and $Δ^k_h$ denote the $k${\rm th} order difference. In this article, under some mild extra assumptions about $X$, the authors prove that, for both parameters $q$ and $γ$ in \emph{sharp} ranges which are related to $X$ and for any locally integrable function $f$ on ${\mathbb{R}^n}$ satisfying $|\nabla^k f|\in X$, $$ \sup_{λ\in(0,\infty)}λ\left\|\left[\int_{\{h\in\mathbb{R}^n:\ |Δ_h^k f(\cdot)|>λ|h|^{k+\fracγ{q}}\}} \left|h\right|^{γ-n}\,dh\right]^\frac{1}{q}\right\|_X \sim \left\|\,\left|\nabla^k f\right|\,\right\|_{X} $$ with the positive equivalence constants independent of $f$. As applications, the authors establish the Brezis--Seeger--Van Schaftingen--Yung (for short, BSVY) characterization of higher-order homogeneous ball Banach Sobolev spaces and higher-order fractional Gagliardo--Nirenberg and Sobolev type inequalities in critical cases. All these results are of quite wide generality and can be applied to various specific function spaces; moreover, even when $X:= L^{q}$, these results when $k=1$ coincide with the best known results and when $k\ge 2$ are completely new. The first novelty is to establish a sparse characterization of dyadic cubes in level sets related to the higher-order local approximation, which, together with the well-known Whitney inequality in approximation theory, further induces a higher-order weighted variant of the remarkable inequality obtained by A. Cohen, W. Dahmen, I. Daubechies, and R. DeVore; the second novelty is to combine this weighted inequality neatly with a variant higher-order Poincaré inequality to establish the desired upper estimate of BSVY formulae in weighted Lebesgue spaces.

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arXiv2025

Difference and Wavelet Characterizations of Distances from Functions in Lipschitz Spaces to Their Subspaces

Let $Λ_s$ denote the Lipschitz space of order $s\in(0,\infty)$ on $\mathbb{R}^n$, which consists of all $f\in\mathfrak{C}\cap L^\infty$ such that, for some constant $L\in(0,\infty)$ and some integer $r\in(s,\infty)$, \begin{equation*} \label{0-1}Δ_r f(x,y): =\sup_{|h|\leq y} |Δ_h^r f(x)|\leq L y^s, \ x\in\mathbb{R}^n, \ y \in(0, 1]. \end{equation*} Here (and throughout the article) $\mathfrak{C}$ refers to continuous functions, and $Δ_h^r$ is the usual $r$-th order difference operator with step $h\in\mathbb{R}^n$. For each $f\in Λ_s$ and $\varepsilon\in(0,L)$, let $ S(f,\varepsilon):= \{ (x,y)\in\mathbb{R}^n\times [0,1]: \frac {Δ_r f(x,y)}{y^s}>\varepsilon\}$, and let $μ: \mathcal{B}(\mathbb{R}_+^{n+1})\to [0,\infty]$ be a suitably defined nonnegative extended real-valued function on the Borel $σ$-algebra of subsets of $\mathbb{R}_+^{n+1}$. Let $\varepsilon(f)$ be the infimum of all $\varepsilon\in(0,\infty)$ such that $μ(S(f,\varepsilon))<\infty$. The main target of this article is to characterize the distance from $f$ to a subspace $V\cap Λ_s$ of $Λ_s$ for various function spaces $V$ (including Sobolev, Besov--Triebel--Lizorkin, and Besov--Triebel--Lizorkin-type spaces) in terms of $\varepsilon(f)$, showing that \begin{equation*} \varepsilon(f)\sim \mathrm{dist} (f, V\cap Λ_s)_{Λ_s}: = \inf_{g\in Λ_s\cap V} \|f-g\|_{Λ_s}.\end{equation*} Moreover, we present our results in a general framework based on quasi-normed lattices of function sequences $X$ and Daubechies $s$-Lipschitz $X$-based spaces.

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arXiv2025

Matrix-Weighted Besov--Triebel--Lizorkin Spaces of Optimal Scale: Boundedness of Pseudo-Differential, Trace, and Calderón--Zygmund Operators

This article is a continuation of our work on generalized matrix-weighted Besov--Triebel--Lizorkin-type spaces with matrix $\mathcal{A}_{\infty}$ weights. In this article, we establish the boundedness of pseudo-differential, trace, and Calderón--Zygmund operators on these spaces. The main tools involved in this article are the molecular and the wavelet characterizations of these spaces. Since generalized matrix-weighted Besov--Triebel--Lizorkin-type spaces include many classical function spaces such as matrix-weighted Besov--Triebel--Lizorkin spaces, all the results in this article are of wide generality.

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arXiv2025

Matrix-Weighted Besov-Triebel-Lizorkin Spaces of Optimal Scale: Real-Variable Characterizations, Invariance on Integrable Index, and Sobolev-Type Embedding

In this article, using growth functions we introduce generalized matrix-weighted Besov-Triebel-Lizorkin-type spaces with matrix $\mathcal{A}_{\infty}$ weights. We first characterize these spaces, respectively, in terms of the $φ$-transform, the Peetre-type maximal function, and the Littlewood-Paley functions. Furthermore, after establishing the boundedness of almost diagonal operators on the corresponding sequence spaces, we obtain the molecular and the wavelet characterizations of these spaces. As applications, we find the sufficient and necessary conditions for the invariance of those Triebel-Lizorkin-type spaces on the integrable index and also for the Sobolev-type embedding of all these spaces. The main novelty exists in that these results are of wide generality, the growth condition of growth functions is not only sufficient but also necessary for the boundedness of almost diagonal operators and hence this new framework of Besov-Triebel-Lizorkin-type is optimal, some results either are new or improve the known ones even for known matrix-weighted Besov-Triebel-Lizorkin spaces, and, furthermore, even in the scalar-valued setting, all the results are also new.

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arXiv2025

Parabolic Extrapolation and Its Applications to Characterizing Parabolic BMO Spaces via Parabolic Fractional Commutators

In this article, we establish the parabolic version of the celebrated Rubio de Francia extrapolation theorem. As applications, we obtain new characterizations of parabolic BMO-type spaces in terms of various commutators of parabolic fractional operators with time lag. The key tools to achieve these include to establish the appropriate form in the parabolic setting of the parabolic Rubio de Francia iteration algorithm, the Cauchy integral trick, and a modified Fourier series expansion argument adapted to the parabolic geometry. The novelty of these results lies in the fact that, for the first time, we not only introduce a new class of commutators associated with parabolic fractional integral operators with time lag, but also utilize them to provide a characterization of the parabolic BMO-type space in the high-dimensional case.

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arXiv2025

Refining Image Edge Detection via Linear Canonical Riesz Transforms

Combining the linear canonical transform and the Riesz transform, we introduce the linear canonical Riesz transform (for short, LCRT), which is further proved to be a linear canonical multiplier. Using this LCRT multiplier, we conduct numerical simulations on images. Notably, the LCRT multiplier significantly reduces the complexity of the algorithm. Based on these we introduce the new concept of the sharpness $R^{\rm E}_{\rm sc}$ of the edge strength and continuity of images associated with the LCRT and, using it, we propose a new LCRT image edge detection method (for short, LCRT-IED method) and provide its mathematical foundation. Our experiments indicate that this sharpness $R^{\rm E}_{\rm sc}$ characterizes the macroscopic trend of edge variations of the image under consideration, while this new LCRT-IED method not only controls the overall edge strength and continuity of the image, but also excels in feature extraction in some local regions. These highlight the fundamental differences between the LCRT and the Riesz transform, which are precisely due to the multiparameter of the former. This new LCRT-IED method might be of significant importance for image feature extraction, image matching, and image refinement.

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arXiv2025

Maximal Function and Atomic Characterizations of Matrix-Weighted Hardy Spaces with Their Applications to Boundedness of Calderón--Zygmund Operators

Let $p\in(0,1]$ and $W$ be an $A_p$-matrix weight, which in scalar case is exactly a Muckenhoupt $A_1$ weight. In this article, we introduce matrix-weighted Hardy spaces $H^p_W$ via the matrix-weighted grand non-tangential maximal function and characterize them, respectively, in terms of various other maximal functions and atoms, both of which are closely related to matrix weights under consideration and their corresponding reducing operators. As applications, we first establish the finite atomic characterization of $H^p_W$, then using it we give a criterion on the boundedness of sublinear operators from $H^p_W$ to any $γ$-quasi-Banach space, and finally applying this criterion we further obtain the boundedness of Calderón--Zygmund operators on $H^p_W$. The main novelty of these results lies in that the aforementioned maximal functions related to reducing operators are new even in the scalar weight case and we characterize these matrix-weighted Hardy spaces by a fresh and natural variant of classical weighted atoms via first establishing a Calderón--Zygmund decomposition which is also new even in the scalar weight case.

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arXiv2025

Self-improving properties of weighted norm inequalities on metric measure spaces

This work discusses self-improving properties of the Muckenhoupt condition and weighted norm inequalities for the Hardy-Littlewood maximal function on metric measure spaces with a doubling measure. Our main result provides direct proofs of these properties by applying a Whitney covering argument and a technique inspired by the Calderón-Zygmund decomposition. In particular, this approach does not rely on reverse Hölder inequalities.

math.CA↗

arXiv2025

Besov--Triebel--Lizorkin-Type Spaces with Matrix $A_\infty$ Weights

Introduced by A. Volberg, matrix $A_{p,\infty}$ weights provide a suitable generalization of Muckenhoupt $A_\infty$ weights from the classical theory. In our previous work, we established new characterizations of these weights. Here, we use these results to study inhomogeneous Besov-type and Triebel--Lizorkin-type spaces with such weights. In particular, we characterize these spaces, in terms of the $φ$-transform, molecules, and wavelets, and obtain the boundedness of almost diagonal operators, pseudo-differential operators, trace operators, pointwise multipliers, and Calderón--Zygmund operators on these spaces. This is the first systematic study of inhomogeneous Besov--Triebel--Lizorkin-type spaces with $A_{p,\infty}$-matrix weights, but some of the results are new even when specialized to the scalar unweighted case.

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arXiv2024

Sharp Poincaré--Sobolev Inequalities of Choquet--Lorentz Integrals with Respect to Hausdorff Contents on Bounded John Domains

Let $Ω$ be a bounded John domain in $\mathbb R^n$ with $n\ge 2$, and let $\mathcal{H}_{\infty }^δ$ denote the Hausdorff content of dimension $δ\in (0,n]$. In this article, the authors prove the Poincaré and the Poincaré--Sobolev inequalities, with sharp ranges of indices, on Choquet--Lorentz integrals with respect to $\mathcal{H}_{\infty }^δ$ for all continuously differentiable functions on $Ω$. These results not only extend the recent Poincaré and Poincaré--Sobolev inequalities to the Choquet--Lorentz integrals, but also provide some endpoint estimates (weak type) in the critical case. One of the main novelties exists in that, to achieve the goals, the authors develop some new tools associated with Choquet--Lorentz integrals on $\mathcal{H}_{\infty }^δ$, such as the fractional Hardy--Littlewood maximal inequality and the Hedberg-type pointwise estimate on the Riesz potential. As an application, the authors obtain the sharp boundedness of the Riesz potential on Choquet--Lorentz integrals. Moreover, even for classical Lorentz integrals, these Poincaré and Poincaré--Sobolev inequalities are also new.

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arXiv2024

Regularity of Characteristic Functions in Besov-Type and Triebel--Lizorkin-Type Spaces

In this article, the authors determine the optimal regularity of characteristic functions in Besov-type and Triebel--Lizorkin-type spaces under restrictions on the measure of the $δ$-neighborhoods of the boundary. In particular, the necessary and sufficient conditions for the membership in these spaces of characteristic functions of the snowflake domain and also some spiral type domains are obtained.

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