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David Cruz-Uribe

Publications and source records attributed to David Cruz-Uribe.

At least 19 recordsLinked to original sources
arXiv2026

Degenerate Sobolev and Poincar\'e inequalities via extrapolation

In this paper we prove matrix weighted Sobolev and Poincar\'e inequalities using techniques derived from the theory of Rubio de Francia extrapolation. Given weights $w,\,v$ and a symmetric non-negative definite matrix valued function $Q$ defined on a connected open subset $\Omega$ of $\mathbb{f}R^n$ that satisfies the lower ellipticity condition \[ w(x)^p \leq |\sqrt{Q(x)}\xi|^p,\quad \xi\in \mathbb{R}^n, \] we give Lebesgue integrability conditions on the weights $w,v$ that ensure there exists $\tau\geq 1$ so that Sobolev and Poincar\'e inequalities of the form \[\bigg(\int_\Omega |u|^{\tau p} \,vdx\bigg)^{\frac{1}{\tau p}} \leq C(v,w) \bigg(\int_\Omega |\sqrt{Q}\nabla u|^p\,dx\bigg)^{\frac{1}{ p}},\textrm{ and}\] \[\bigg(\int_\Omega |u-\langle u\rangle_{\Omega,v}|^{\tau p} \,v dx\bigg)^\frac{1}{\tau p} \leq C(v,w)\bigg(\int_\Omega|\sqrt{Q}\nabla u|^p \, dx\bigg)^{\frac{1}{p}}\] hold for smooth $u$. We explore these and related results in the context of several examples that include John domains, the Heisenberg group, and CR manifolds.

math.AP↗
arXiv2026

On off-diagonal operators in matrix-weighted spaces

In this paper we prove matrix-weighted inequalities for fractional operators and their commutators. We do so by developing the theory of convex body domination for such operators. Using this approach we prove quantitative estimates for the fractional integral operator (or Riesz potential) and its commutators, and prove matrix-weighted Gagliardo-Nirenberg-Sobolev inequalities for vector-valued functions.

math.CA↗
arXiv2025

The Stein-Weiss inequality in variable exponent Morrey spaces

In this paper we prove the Stein-Weiss inequality in variable exponent Morrey spaces over a bounded domain. Our work extends earlier results in the variable exponent Lebesgue and Morrey settings, and utilizes new proof techniques applicable to Morrey spaces. We build on the foundational paper by Almeida, Hasanov, and Samko, which introduced Morrey spaces of variable exponents. As an application of our main result, we prove Poincar\'e-type inequalities using the approach of a recent paper by the first and third authors.

math.CA↗
arXiv2025

Recent results on matrix weighted norm inequalities

In this paper we give an overview of recent work on matrix weights, with particular emphasis on convex body sparse domination for singular integrals, Rubio de Francia extrapolation, and Jones factorization. To provide context and motivation, we survey the comparable results in the scalar weighted case.

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arXiv2025

Existence and uniqueness of solutions of degenerate elliptic equations with lower order terms

We prove the existence and uniqueness of solutions to a Dirichlet problem \[ \begin{cases} Lu = f + v^{-1}\text{Div}(v{\bf e} h), & x \in \Omega; u = 0, & x \in \partial \Omega, \end{cases}\] where $L$ is a degenerate, linear, second order elliptic operator with lower order terms. We assume very weak hypotheses, in terms of the coefficients of the equation, and we also assume the existence of degenerate Sobolev and Poincar\'e inequalities. One notable feature of our result is that we show that we can assume significantly weaker versions of the Sobolev inequality if we in turn assume stronger integrability conditions on the coefficients. Our theorems generalize a number of results in the literature on degenerate elliptic equations.

math.AP↗
arXiv2025

Off-diagonal matrix extrapolation for Muckenhoupt bases

In this paper we extend the theory of Rubio de Francia extrapolation for matrix weights, recently introduced by Bownik and the first author, to off-diagonal extrapolation. We also show that the theory of matrix weighted extrapolation can be extended to matrix $\mathcal{A}_p$ classes defined with respect to a general basis, provided that a version of the Christ-Goldberg maximal operator is assumed to be bounded. Finally, we extend a recent result by Vuorinen and show that all of the multiparameter bases have this property.

math.CA↗
arXiv2024

Bounded solutions of degenerate elliptic equations with an Orlicz-gain Sobolev inequality

We consider the boundedness and exponential integrability of solutions to the Dirichlet problem for the degenerate elliptic equation \[ -v^{-1}\mathrm{Div}(|\sqrt{Q}\nabla u|^{p-2}Q\nabla u)=f|f|^{p-2}- v^{-1}\mathrm{Div}(v|g|^{p-2}g \mathbf{t}), \quad 1 1$. In our results we study the interplay between the Sobolev inequality and the regularity assumptions needed on $f$ and $g$ to prove that the solution is bounded or is exponentially integrable. Our results generalize those previously proved in previous work by the authors.

math.AP↗
arXiv2024

The reverse H\"older inequality for $\mathcal{A}_{p(\cdot)}$ weights with applications to matrix weights

In this paper we prove a reverse H\"{o}lder inequality for the variable exponent Muckenhoupt weights $\mathcal{A}_{p(\cdot)}$, introduced by the first author, Fiorenza, and Neugeabauer. All of our estimates are quantitative, showing the dependence of the exponent function on the $\mathcal{A}_{p(\cdot)}$ characteristic. As an application, we use the reverse H\"{o}lder inequality to prove that the matrix $\mathcal{A}_{p(\cdot)}$ weights, introduced in our previous paper, have both a right and left-openness property. This result is new even in the scalar case.

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arXiv2024

Bounded weak solutions with Orlicz space data: an overview

It is well known that non-negative solutions to the Dirichlet problem $\Delta u =f$ in a bounded domain $\Omega$, where $f\in L^q(\Omega)$, $q>\frac{n}2$, satisfy $\|u\|_{L^\infty(\Omega)} \leq C\|f\|_{L^q(\Omega)}$. We generalize this result by replacing the Laplacian with a degenerate elliptic operator, and we show that we can take the data $f$ in an Orlicz space $L^A(\Omega)$ that, in the classical case, lies strictly between $L^{\frac{n}{2}}(\Omega)$ and $L^q(\Omega)$, $q>\frac{n}2$.

math.AP↗
arXiv2024

Two-weight norm inequalities for parabolic fractional maximal functions

We prove two-weight norm inequalities for parabolic fractional maximal functions using parabolic Muckenhoupt weights. In particular, we prove a two-weight, weak-type estimate and Fefferman-Stein type inequalities for the centered parabolic maximal function. We also prove that a parabolic Sawyer-type condition implies the strong-type estimate for the parabolic fractional maximal function. Finally, we prove the strong-type estimate for the centered parabolic maximal function assuming a stronger parabolic Muckenhoupt bump condition.

math.CA↗
arXiv2024

Poincaré and Sobolev inequalities with variable exponents and log-Holder continuity only at the boundary

We prove Sobolev-Poincaré and Poincaré inequalities in variable Lebesgue spaces $L^{p(\cdot)}(Ω)$, with $Ω\subset{\mathbb R}^n$ a bounded John domain, with weaker regularity assumptions on the exponent $p(\cdot)$ that have been used previously. In particular, we require $p(\cdot)$ to satisfy a new \emph{boundary $\log$-Hölder condition} that imposes some logarithmic decay on the oscillation of $p(\cdot)$ towards the boundary of the domain. Some control over the interior oscillation of $p(\cdot)$ is also needed, but it is given by a very general condition that allows $p(\cdot)$ to be discontinuous at every point of $Ω$. Our results follows from a local-to-global argument based on the continuity of certain Hardy type operators. We provide examples that show that our boundary $\log$-Hölder condition is essentially necessary for our main results. The same examples are adapted to show that this condition is not sufficient for other related inequalities. Finally, we give an application to a Neumann problem for a degenerate $p(\cdot)$-Laplacian.

math.AP↗
arXiv2024

Necessary conditions for the boundedness of fractional operators on variable Lebesgue spaces

In this paper we prove necessary conditions for the boundedness of fractional operators on the variable Lebesgue spaces. More precisely, we find necessary conditions on an exponent function $\pp$ for a fractional maximal operator $M_α$ or a non-degenerate fractional singular integral operator $T_α$, $0 \leq α< n$, to satisfy weak $(\pp,\qq)$ inequalities or strong $(\pp,\qq)$ inequalities, with $\qq$ being defined pointwise almost everywhere by % \[ \frac{1}{p(x)} - \frac{1}{q(x)} = \fracα{n}. \] % We first prove preliminary results linking fractional averaging operators and the $K_0^α$ condition, a qualitative condition on $\pp$ related to the norms of characteristic functions of cubes, and show some useful implications of the $K_0^α$ condition. We then show that if $M_α$ satisfies weak $(\pp,\qq)$ inequalities, then $\pp \in K_0^α(\R^n)$. We use this to prove that if $M_α$ satisfies strong $(\pp,\qq)$ inequalities, then $p_->1$. Finally, we prove a powerful pointwise estimate for $T_α$ that relates $T_α$ to $M_α$ along a carefully chosen family of cubes. This allows us to prove necessary conditions for fractional singular integral operators similar to those for fractional maximal operators.

math.CA↗
arXiv2024

On the embedding between the variable Lebesgue space $L^{p(\cdot)}(Ω)$ and the Orlicz space $L(\log L)^α(Ω)$

We give a sharp sufficient condition on the distribution function, $|\{x\in Ω:\,p(x)\leq 1+λ\}|$, $λ>0$, of the exponent function $p(\cdot): Ω\to [1,\infty)$ that implies the embedding of the variable Lebesgue space $L^{p(\cdot)}(Ω)$ into the Orlicz space $L(\log L)^α(Ω)$, $α>0$, where $Ω$ is an open set with finite Lebesgue measure. As applications of our results, we first give conditions that imply the strong differentiation of integrals of functions in $L^{p(\cdot)}((0,1)^{n})$, $n>1$. We then consider the integrability of the maximal function on variable Lebesgue spaces, where the exponent function $p(\cdot)$ approaches $1$ in value on some part of the domain. This result is an improvement of the result in~\cite{CUF2}.

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arXiv2024

The Ross-Darboux-Stieltjes Integral

Motivated by the limitations of the traditional definitions of the Riemann-Stieltjes and Darboux-Stieltjes integrals, we introduce a generalized Darboux-Stieltjes integral that is equivalent to an earlier generalization by Ross \cite{Ross}. Our definition builds upon an approach to the Darboux-Stieltjes integral recently introduced by the first author and Convertito \cite{TSI}. We show that our definition agrees with all previous definitions, but that the class of integrable functions is much larger. We develop all the analogs of the classic results for the Riemann integral, and rectify the problems inherent in the definition of the Darboux-Stieltjes integral in \cite{TSI}. In particular, we show the Bounded Convergence Theorem holds for our definition and that it agrees with the Lebesgue-Stieltjes integral.

math.CA↗
arXiv2024

Degenerate parabolic $p$-Laplacian equations: existence, uniqueness and asymptotic behavior of solutions

In this paper we study the degenerate parabolic $p$-Laplacian,$ \partial_t u - v^{-1}{\rm div}(|\sqrt{Q} \nabla u|^{p-2} Q \nabla u)=0$, where the degeneracy is controlled by a matrix $Q$ and a weight $v$. With mild integrability assumptions on $Q$ and $v$, we prove the existence and uniqueness of solutions on any interval $[0,T]$. If we further assume the existence of a degenerate Sobolev inequality with gain, the degeneracy again controlled by $v$ and $Q$, then we can prove both finite time extinction and ultracontractive bounds. Moreover, we show that there is equivalence between the existence of ultracontractive bounds and the weighted Sobolev inequality.

math.AP↗
arXiv2024

Matrix weights, singular integrals, Jones factorization and Rubio de Francia extrapolation

In this article we give an overview of the problem of finding sharp constants in matrix weighted norm inequalities for singular integrals, the so-called matrix A2 conjecture. We begin by reviewing the history of the problem in the scalar case, including a sketch of the proof of the scalar A2 conjecture. We then discuss the original, qualitative results for singular integrals with matrix weights and the best known quantitative estimates. We give an overview of new results by the author and Bownik, who developed a theory of harmonic analysis on convex set-valued functions. This led to the proof the Jones factorization theorem and the Rubio de Francia extrapolation theorem for matrix weights, two longstanding problems. Rubio de Francia extrapolation was expected to be a major tool in the proof of the matrix A2 conjecture; however, this conjecture was very recently proved false. We discuss this problem.

math.CA↗
arXiv2023

Weighted weak-type inequalities for maximal operators and singular integrals

We prove quantitative, one-weight, weak-type estimates for maximal operators, singular integrals, fractional maximal operators and fractional integral operators. We consider a kind of weak-type inequality that was first studied by Muckenhoupt and Wheeden and later by Cruz-Uribe, Martell and Perez. We obtain quantitative estimates for these operators in both the scalar and matrix weighted setting using sparse domination techniques. Our results extend those obtained by Cruz-Uribe, Isralowitz, Moen, Pott, and Rivera-Ríos for singular integrals and maximal operators when $p=1$.

math.CA↗