arXiv subjects

Blair Davey

Publications and source records attributed to Blair Davey.

At least 19 recordsLinked to original sources

arXiv2026

Unique continuation at infinity for Schrödinger equations with Reverse Hölder Potentials

In this article, we study unique continuation properties at infinity for solutions to generalized Schrödinger equations with potential functions that belong to the reverse Hölder class. For equations of the form $-div (A \nabla u) + V u = 0$ in $\mathbb{R}^n$, where $A$ is bounded and elliptic and $V \in RH_p$ for some $p \in [\frac n 2, \infty]$, we prove that if a solution doesn't grow too quickly at infinity, then it must be trivial. We use $d_V$, the Agmon distance function associated to $V$, to quantify the threshold growth rate. More precisely, there exists a constant $γ_0 > 0$ so that if $|u(x)| \lesssim \exp(γd_V(x, 0))$ for some $γ< γ_0$ and every $x \in \mathbb{R}^n$, then $u$ must be trivial. The result may be interpreted as a Liouville-type theorem and is related to Landis' conjecture. Our proof techniques are inspired by Z. Shen's exponential decay estimates for fundamental solutions of Schrödinger operators and involve the application of a Fefferman-Phong inequality.

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arXiv2026

Fractional Parabolic Theory as a High-Dimensional Limit of Fractional Elliptic Theory

This paper continues the program that was initiated in \cite{Dav18} and continued in \cite{DSVG24}, where a high-dimensional limiting technique was developed and used to prove certain parabolic theorems from their elliptic counterparts. The articles \cite{Dav18} and \cite{DSVG24} address the constant-coefficient and variable-coefficient settings, respectively. Here, we focus on fractional operators. As shown in \cite{CS07}, \cite{NS16}, \cite{ST17}, fractional operators may be associated with certain degenerate operators via extension problems, so we study the corresponding class of degenerate operators. Our high-dimensional limiting technique is demonstrated through new proofs of three theorems for degenerate parabolic equations. Specifically, we establish the monotonicity of Almgren-type, Weiss-type, and Alt-Caffarelli-Friedman-type functionals in the degenerate parabolic setting. Each new parabolic proof in this article is based on a (new) related elliptic theorem and a careful limiting argument that is reminiscent of those from \cite{Dav18} and \cite{DSVG24}. Our proof of the degenerate parabolic Weiss-type monotonicity formula additionally uses an epiperimetric inequality for weakly $a$-harmonic functions, which we also prove. To the best of our knowledge, our Alt-Caffarelli-Friedman monotonicity result is new.

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arXiv2026

Optimal rates of decay at infinity for solutions to Schrödinger equations

We prove rates of decay at infinity for solutions to variable-coefficient Schrödinger equations of the form $-\text{div}(A \nabla u) + W \cdot \nabla u + V u = λu$ in cylinders, $\mathbb{T}^d \times \mathbb{R}^m$. We assume that $W$ and $V$ are bounded and that $λ\in \mathbb{C}$. Our rates depend on the decay of $|\nabla A|$ at infinity. In particular, we prove a range of quantitative unique continuation-type results at infinity when $|\nabla A(θ, x)| \le C (1 + |x|)^{-τ}$ for $τ\in [0,1]$. By adapting the methods in [KLP25], we construct explicit solutions to demonstrate the sharpness of our estimates for each such $τ$.

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arXiv2026

A frequency function approach to quantitative unique continuation for elliptic equations

We investigate the quantitative unique continuation properties of solutions to second-order elliptic equations with lower-order terms. In particular, we establish quantitative forms of the strong unique continuation property for solutions to generalized Schrödinger equations of the form $- \text{div}(A \nabla u) + W \cdot \nabla u + V u = 0$, where we assume that $A$ is bounded, elliptic, symmetric, and Lipschitz continuous, while $W$ belongs to $L^\infty$ and $V$ belongs to $L^p$ for some $p \ge n$. We also study the global unique continuation properties of solutions to these equations, establishing results that are related to Landis' conjecture concerning the optimal rate of decay at infinity. Versions of the theorems in this article have been previously proved using Carleman estimates, but here we present novel proof techniques that rely on frequency functions.

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arXiv2025

On Landis' conjecture in the plane for real-valued potentials with decay

We investigate the quantitative unique continuation properties of real-valued solutions to planar Schrödinger equations with potential functions that exhibit pointwise decay at infinity. That is, for equations of the form $-Δu + V u = 0$ in $\mathbb{R}^2$, where $|V(z)| \lesssim \langle z \rangle^{-N}$ for some $N > 0$, we prove that real-valued solutions satisfy exponential decay estimates with a rate that depends explicitly on $N$. Examples show that the estimates established here are essentially sharp. The case of $N = 0$ corresponds to the Landis conjecture, which was proved for real-valued solutions in the plane in [LMNN20], while the case of $N < 0$ was previously investigated by the author in [Dav24]. Here, the proof techniques rely on the ideas presented in [LMNN20] combined with conformal transformations and an iteration scheme.

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arXiv2025

Self-similar sets and Lipschitz graphs

We investigate and quantify the distinction between rectifiable and purely unrectifiable 1-sets in the plane. That is, given that purely unrectifiable 1-sets always have null intersections with Lipschitz images, we ask whether these sets intersect with Lipschitz images at a dimension that is close to one. In an answer to this question, we show that one-dimensional attractors of iterated function systems that satisfy the open set condition have subsets of dimension arbitrarily close to one that can be covered by Lipschitz graphs. Moreover, the Lipschitz constant of such graphs depends explicitly on the difference between the dimension of the original set and the subset that intersects with the graph.

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arXiv2023

On Landis' conjecture in the plane for potentials with growth

We investigate the quantitative unique continuation properties of real-valued solutions to Schrödinger equations in the plane with potentials that exhibit growth at infinity. More precisely, for equations of the form $Δu - V u = 0$ in $\mathbb{R}^2$, with $|V(z)| \lesssim |z|^{N}$ for some $N \ge 0$, we prove that real-valued solutions satisfy exponential decay estimates with a rate that depends explicitly on $N$. The case $N = 0$ corresponds to the Landis conjecture, which was proved for real-valued solutions in the plane in [LMNN20]. As such, the results in this article may be interpreted as generalized Landis-type theorems. Our proof techniques rely heavily on the ideas presented in [LMNN20].

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arXiv2023

Variable-coefficient parabolic theory as a high-dimensional limit of elliptic theory

This paper continues the study initiated in [B. Davey, Parabolic theory as a high-dimensional limit of elliptic theory, Arch Rational Mech Anal 228 (2018)], where a high-dimensional limiting technique was developed and used to prove certain parabolic theorems from their elliptic counterparts. In this article, we extend these ideas to the variable-coefficient setting. This generalized technique is demonstrated through new proofs of three important theorems for variable-coefficient heat operators, one of which establishes a result that is, to the best of our knowledge, also new. Specifically, we give new proofs of $L^2 \to L^2$ Carleman estimates and the monotonicity of Almgren-type frequency functions, and we prove a new monotonicity of Alt-Caffarelli-Friedman-type functions. The proofs in this article rely only on their related elliptic theorems and a limiting argument. That is, each parabolic theorem is proved by taking a high-dimensional limit of a related elliptic result.

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arXiv2022

Exponential Decay Estimates for Fundamental Matrices of Generalized Schrödinger Systems

In this article, we investigate systems of generalized Schrödinger operators and their fundamental matrices. More specifically, we establish the existence of such fundamental matrices and then prove sharp upper and lower exponential decay estimates for them. The Schrödinger operators that we consider have leading coefficients that are bounded and uniformly elliptic, while the zeroth-order terms are assumed to be nondegenerate and belong to a reverse Hölder class of matrices. In particular, our operators need not be self-adjoint. The exponential bounds are governed by the so-called upper and lower Agmon distances associated to the reverse Hölder matrix that serves as the potential function. Furthermore, we thoroughly discuss the relationship between this new reverse Hölder class of matrices, the more classical matrix $\mathcal{A}_{p,\infty}$ class, and the matrix $\mathcal{A}_{\infty}$ class introduced in [Dall15].

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arXiv2021

A Quantification of a Besicovitch Nonlinear Projection Theorem via Multiscale Analysis

The Besicovitch projection theorem states that if a subset $E$ of the plane has finite length in the sense of Hausdorff measure and is purely unrectifiable (so its intersection with any Lipschitz graph has zero length), then almost every orthogonal projection of $E$ to a line will have zero measure. In other words, the Favard length of a purely unrectifiable $1$-set vanishes. In this article, we show that when linear projections are replaced by certain nonlinear projections called curve projections, this result remains true. In fact, we go further and use multiscale analysis to prove a quantitative version of this Besicovitch nonlinear projection theorem. Roughly speaking, we show that if a subset of the plane has finite length in the sense of Hausdorff and is nearly purely unrectifiable, then its Favard curve length is very small. Our techniques build on those of Tao, who in [Tao09] proves a quantification of the original Besicovitch projection theorem.

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arXiv2021

Upper and lower bounds on the rate of decay of the Favard curve length for the four-corner Cantor set

The Favard length of a subset of the plane is defined as the average of its orthogonal projections. This quantity is related to the probabilistic Buffon needle problem; that is, the Favard length of a set is proportional to the probability that a needle or a line that is dropped at random onto the set will intersect the set. If instead of dropping lines onto a set, we drop fixed curves, then the associated Buffon curve probability is proportional to the so-called Favard curve length. As we show in our companion paper, a Besicovitch generalized projection theorem still holds in the setting where lines are replaced by curves. Consequently, the Favard curve length of any purely unrectifiable set is zero. Since the four-corner Cantor set is a compact, purely unrectifiable $1$-set with bounded, non-zero Hausdorff measure, then its Favard curve length equals zero. In this article, we estimate upper and lower bounds for the rate of decay of the Favard curve length of the four-corner Cantor set. Our techniques build on the ideas that have been previously used for the classical Favard length.

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arXiv2020

Improved quantitative unique continuation for complex-valued drift equations in the plane

In this article, we investigate the quantitative unique continuation properties of complex-valued solutions to drift equations in the plane. We consider equations of the form $Δu + W \cdot \nabla u = 0$ in $\mathbb{R}^2$, where $W = W_1 + i W_2$ with each $W_j$ real-valued. Under the assumptions that $W_j \in L^{q_j}$ for some $q_1 \in [2, \infty]$, $q_2 \in (2, \infty]$, and $W_2$ exhibits rapid decay at infinity, we prove new global unique continuation estimates. This improvement is accomplished by reducing our equations to vector-valued Beltrami systems. Our results rely on a novel order of vanishing estimate combined with a finite iteration scheme.

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arXiv2019

On Landis' conjecture in the plane for some equations with sign-changing potentials

In this article, we investigate the quantitative unique continuation properties of real-valued solutions to elliptic equations in the plane. Under a general set of assumptions on the operator, we establish quantitative forms of Landis' conjecture. Of note, we prove a version of Landis' conjecture for solutions to $-Δu + V u = 0$, where $V$ is a bounded function whose negative part exhibits polynomial decay at infinity. The main mechanism behind the proofs is an order of vanishing estimate in combination with an iteration scheme. To prove the order of vanishing result, we present a new idea for constructing positive multipliers and use it reduce the equation to a Beltrami system. The resulting first-order equation is analyzed using the similarity principle and the Hadamard three-quasi-circle theorem.

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arXiv2019

Quantitative unique continuation for Schrödinger operators

We investigate the quantitative unique continuation properties of solutions to second order elliptic equations with singular lower order terms. The main theorem presents a quantification of the strong unique continuation property for $Δ+ V$. That is, for any non-trivial $u$ that solves $Δu + V u = 0$ in some open, connected subset of $\mathbb{R}^n$, we estimate the vanishing order of solutions in terms of the $L^t$-norm of $V$. Our results apply to all $t > \frac n 2$ and $n \ge 3$. With these maximal order of vanishing estimates, we employ a scaling argument to produce quantitative unique continuation at infinity estimates for global solutions to $Δu + V u = 0$. To handle $V \in L^t$ for every $t \in (\frac n 2, \infty]$, we prove a novel $L^p - L^q$ Carleman estimate by interpolating a known $L^p - L^2$ estimate with a new endpoint Carleman estimate. This new Carleman estimate may also be used to establish improved order of vanishing estimates for equations with a first order term, those of the form $Δu + W \cdot \nabla u + V u = 0$.

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arXiv2018

On Landis' conjecture in the plane when the potential has an exponentially decaying negative part

In this article, we continue our investigation into the unique continuation properties of real-valued solutions to elliptic equations in the plane. More precisely, we make another step towards proving a quantitative version of Landis' conjecture by establishing unique continuation at infinity estimates for solutions to equations of the form $- Δu + V u = 0$ in $\mathbb{R}^2$, where $V = V_+ - V_-$, $V_+ \in L^\infty$, and $V_-$ is a non-trivial function that exhibits exponential decay at infinity. The main tool in the proof of this theorem is an order of vanishing estimate in combination with an iteration scheme. To prove the order of vanishing estimate, we establish a similarity principle for vector-valued Beltrami systems.

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arXiv2018

Landis' conjecture for general second order elliptic equations with singular lower order terms in the plane

In this article, we study the order of vanishing and a quantitative form of Landis' conjecture in the plane for solutions to second-order elliptic equations with variable coefficients and singular lower order terms. Precisely, we let $A$ be real-valued, bounded and elliptic, but not necessary symmetric or continuous, and we assume that $V$ and $W_i$ are real-valued and belong to $L^p$ and $L^{q_i}$, respectively. We prove that if $u$ is a real-valued, bounded and normalized solution to an equation of the form $-\nabla \cdot (A \nabla u + W_1 u) + W_2 \cdot \nabla u + V u = 0$ in $B_d$, then under suitable conditions on the lower order terms, for any $r$ sufficiently small, the following order of vanishing estimate holds $$\|u\|_{L^\infty(B_r)} \ge r^{C M},$$ where $M$ depends on the Lebesgue norms of the lower order terms. In a number of settings, a scaling argument gives rise to a quantitative form of Landis' conjecture, \[ \inf_{|z_0| = R} \|u\|_{L^\infty(B_1(z_0))} \ge \exp(- C R^β\log R), \] where $β$ depends on $p$, $q_1$, and $q_2$. The integrability assumptions that we impose on $V$ and $W_i$ are nearly optimal in view of a scaling argument. We use the theory of elliptic boundary value problems to establish the existence of positive multipliers associated to the elliptic equation. Then the proofs rely on transforming the equations to Beltrami systems and applying a generalization of Hadamard's three-circle theorem.

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arXiv2017

Parabolic theory as a high-dimensional limit of elliptic theory

The aim of this article is to show how certain parabolic theorems follow from their elliptic counterparts. This technique is demonstrated through new proofs of five important theorems in parabolic unique continuation and the regularity theory of parabolic equations and geometric flows. Specifically, we give new proofs of an $L^2$ Carleman estimate for the heat operator, and the monotonicity formulas for the frequency function associated to the heat operator, the two-phase free boundary problem, the flow of harmonic maps, and the mean curvature flow. The proofs rely only on the underlying elliptic theorems and limiting procedures belonging essentially to probability theory. In particular, each parabolic theorem is proved by taking a high-dimensional limit of the related elliptic result.

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arXiv2017

Quantitative uniqueness of solutions to second order elliptic equations with singular lower order terms

In this article, we study the quantitative uniqueness of solutions to second order elliptic equations with singular lower order terms. We quantify the strong unique continuation property by estimating the maximal vanishing order of solutions. That is, when $u$ is a non-trivial solution to $\triangle u + W \cdot \nabla u + V u = 0$ in some open, connected subset of $\mathbb R^n$, where $n \geq 3$, we characterize the vanishing order of solutions in terms of the norms of $V$ and $W$ in their respective Lebesgue spaces. Using these maximal order of vanishing estimates, we also establish quantitative unique continuation at infinity results for solutions to $\triangle u + W \cdot \nabla u + V u = 0$ in $\mathbb R^n$. The main tools in our work are new versions of $L^p\to L^q$ Carleman estimates for a range of $p$- and $q$-values.

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