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Richard S. Laugesen

Publications and source records attributed to Richard S. Laugesen.

At least 19 recordsLinked to original sources
arXiv2026

Elliptic rearrangement for Poisson equations with mean-zero data

The $L^p$ norm of a sign-changing Dirichlet solution of Poisson's equation is shown to be bounded by the maximum of the $L^p$ norms of the ball solutions obtained by signed rearrangement of the source and its negative, assuming the source term has mean value zero. This paired comparison on bounded open sets extends Talenti's elliptic rearrangement principle to solutions that change sign. In dimensions one and two, the paired comparison holds for all $1\le p\le\infty$, and it holds in every dimension for $p=\infty$. The inequalities are sharp and equality cases are characterized. In dimensions $n \geq 3$, paired comparison can fail for every finite $p$ although it remains valid for $p\ge(3n-1)/4$ in the special case of bang-bang sources with mean zero. The mean-zero source assumption is crucial when $p<\infty$. Without it, paired comparison can fail even for bang-bang sources. For sources of arbitrary mean in one dimension, however, the $L^\infty$ paired comparison still holds and so does a comparison for $L^p$ norms of the derivative when $1\le p\le3$.

math.AP↗
arXiv2026

Rayleigh's conjecture for vibrating plates: simpler planar proof and nonlinear extensions

We give a simpler proof of Nadirashvili's theorem (Rayleigh's conjecture) that among vibrating clamped plates of prescribed area in the plane, the disk has the lowest fundamental tone. The proof uses a one-shot Talenti type comparison for sign-changing solutions of the Poisson equation with mean-zero data, established in a companion paper. The comparison considers just two rearranged sources, rather than optimizing over all admissible sources as in the traditional two-ball technique. This ``paired comparison'' simplification yields immediate nonlinear extensions: the disk minimizes the clamped quotient $\|Δu\|_q/\|u\|_p$ among bounded open sets of prescribed area, for every $1\le p,q\le\infty$. Equality is characterized for $q>1$, while exceptional behavior at $q=1$ is identified. In higher dimensions, we prove ball minimality for $p=\infty$ and conjecture minimality for all $p,q$ within the compact embedding range. We also establish the corresponding interval inequality in one dimension.

math.SP↗
arXiv2026

Toward Pólya and Szegő's conjecture for logarithmic vs Newtonian capacity

Pólya and Szegő conjectured in 1945 that the disk achieves the largest electrostatic capacity among all planar sets with given logarithmic capacity. This question generalizes naturally to balls and Newtonian capacities in higher dimensions. The conjecture remains open. This paper establishes a conditional result: the conjecture would follow if among all lower dimensional sets in a horizontal strip with Neumann boundary conditions, the transverse component of the Dirichlet integral of the equilibrium potential could be shown minimal for the lower dimensional ball. As supporting evidence toward this latter property, the paper shows extremality of the ball for $L^p$-norms of the potentials, via Baernstein star-function symmetrization techniques.

math.CA↗
arXiv2026

Riesz energy deformation through insulated strips

For compact sets in Euclidean space, Riesz energies whose exponents differ by $1$ are shown to arise as the endpoint cases of a one-parameter family of infinite-strip energies as the strip thickness increases from $0$ to $\infty$, under Neumann boundary conditions. An approach is suggested to a capacity conjecture of Pólya and Szegő.

math.CA↗
arXiv2025

Spherical caps do not always maximize Neumann eigenvalues on the sphere

We prove the existence of an open set $Ω\subset\mathbb{S}^2$ for which the first positive eigenvalue of the Laplacian with Neumann boundary condition exceeds that of the geodesic disk having the same area. This example holds for large areas and contrasts with results by Bandle and later authors proving maximality of the disk under additional topological or geometric conditions, thereby revealing such conditions to be necessary.

math.AP↗
arXiv2025

Fourth order Saint-Venant inequalities: maximizing compliance and mean deflection among clamped plates

We prove a fourth order analogue of the Saint-Venant inequality: the mean deflection of a clamped plate under uniform transverse load is maximal for the ball, among plates of prescribed volume in any dimension of space. The method works in Euclidean space, hyperbolic space, and the sphere. Similar results for clamped plates under small compression and for the compliance under non-uniform loads are proved to hold in two dimensional Euclidean space, with the higher dimensional and curved cases of those problems left open.

math.AP↗
arXiv2025

Two disks maximize the third Robin eigenvalue: positive parameters

The third eigenvalue of the Robin Laplacian on a simply-connected planar domain of given area is bounded above by the corresponding eigenvalue of a disjoint union of two equal disks, for Robin parameters in $[-4π,4π]$. This sharp inequality was known previously only for negative parameters in $[-4π,0]$, by Girouard and Laugesen. Their proof fails for positive Robin parameters because the second eigenfunction on a disk has non-monotonic radial part. This difficulty is overcome for parameters in $(0,4π]$ by means of a degree-theoretic approach suggested by Karpukhin and Stern that yields suitably orthogonal trial functions.

math.SP↗
arXiv2024

Maximizing Riesz capacity ratios: conjectures and theorems

A shape optimization program is developed for the ratio of Riesz capacities $\text{Cap}_q(K)/\text{Cap}_p(K)$, where $K$ ranges over compact sets in $\mathbb{R}^n$. In different regions of the $pq$-parameter plane, maximality is conjectured for the ball, the vertices of a regular simplex, or the endpoints of an interval. These cases are separated by a symmetry-breaking transition region where the shape of maximizers remains unclear. On the boundary of $pq$-parameter space one encounters existing theorems and conjectures, including: Watanabe's theorem minimizing Riesz capacity for given volume, the classical isodiametric theorem that maximizes volume for given diameter, Szegő's isodiametric theorem maximizing Newtonian capacity for given diameter, and the still-open isodiametric conjecture for Riesz capacity. The first quadrant of parameter space contains Pólya and Szegő's conjecture on maximizing Newtonian over logarithmic capacity for planar sets. The maximal shape for each of these scenarios is known or conjectured to be the ball. In the third quadrant, where both $p$ and $q$ are negative, the maximizers are quite different: when one of the parameters is $-\infty$ and the other is suitably negative, maximality is proved for the vertices of a regular simplex or endpoints of an interval. Much more is proved in dimensions $1$ and $2$, where for large regions of the third quadrant, maximizers are shown to consist of the vertices of intervals or equilateral triangles.

math.CA↗
arXiv2024

Hausdorff measure and decay rate of Riesz capacity

The decay rate of Riesz capacity as the exponent increases to the dimension of the set is shown to yield Hausdorff measure. The result applies to strongly rectifiable sets, and so in particular to submanifolds of Euclidean space. For strictly self-similar fractals, a one-sided decay estimate is found. Along the way, a purely measure theoretic proof is given for subadditivity of the reciprocal of Riesz energy.

math.CA↗
arXiv2024

Riesz capacity: monotonicity, continuity, diameter and volume

Properties of Riesz capacity are developed with respect to the kernel exponent $p \in (-\infty,n)$, namely that capacity is monotonic as a function of $p$, that its endpoint limits recover the diameter and volume of the set, and that capacity is left-continuous with respect to $p$ and is right-continuous provided (when $p \geq 0$) that an additional hypothesis holds. Left and right continuity properties of the equilibrium measure are obtained too.

math.CA↗
arXiv2024

Balls minimize moments of logarithmic and Newtonian equilibrium measures

The $q$-th moment ($q>0$) of electrostatic equilibrium measure is shown to be minimal for a centered ball among $3$-dimensional sets of given capacity, while among $2$-dimensional sets a centered disk is the minimizer for $0<q \leq 2$. Analogous results are developed for Newtonian capacity in higher dimensions and logarithmic capacity in $2$ dimensions. Open problems are raised for Riesz equilibrium moments.

math.CA↗
arXiv2023

Maximizing the second Robin eigenvalue of simply connected curved membranes

The second eigenvalue of the Robin Laplacian is shown to be maximal for a spherical cap among simply connected Jordan domains on the 2-sphere, for substantial intervals of positive and negative Robin parameters and areas. Geodesic disks in the hyperbolic plane similarly maximize the eigenvalue on a natural interval of negative Robin parameters. These theorems extend work of Freitas and Laugesen from the Euclidean case (zero curvature) and the authors' hyperbolic and spherical results for Neumann eigenvalues (zero Robin parameter). Complicating the picture is the numerically observed fact that the second Robin eigenfunction on a large spherical cap is purely radial, with no angular dependence, when the Robin parameter lies in a certain negative interval depending on the cap aperture.

math.SP↗
arXiv2021

Minimizing capacity among linear images of rotationally invariant conductors

Logarithmic capacity is shown to be minimal for a planar set having $N$-fold rotational symmetry ($N \geq 3$), among all conductors obtained from the set by area-preserving linear transformations. Newtonian and Riesz capacities obey a similar property in all dimensions, when suitably normalized linear transformations are applied to a set having irreducible symmetry group. A corollary is Pólya and Schiffer's lower bound on capacity in terms of moment of inertia.

math-ph↗
arXiv2020

Two balls maximize the third Neumann eigenvalue in hyperbolic space

We show that the third eigenvalue of the Neumann Laplacian in hyperbolic space is maximal for the disjoint union of two geodesic balls, among domains of given volume. This extends a recent result by Bucur and Henrot in Euclidean space, while providing a new proof of a key step in their argument

math.SP↗
arXiv2019

Robin spectrum: two disks maximize the third eigenvalue

The third eigenvalue of the Robin Laplacian on a simply-connected planar domain of given area is bounded above by the third eigenvalue of a disjoint union of two disks, provided the Robin parameter lies in a certain range and is scaled in each case by the length of the boundary. Equality is achieved when the domain degenerates suitably to the two disks.

math.SP↗
arXiv2019

The Robin Laplacian - spectral conjectures, rectangular theorems

The first two eigenvalues of the Robin Laplacian are investigated along with their gap and ratio. Conjectures by various authors for arbitrary domains are supported here by new results for rectangular boxes. Results for rectangular domains include that: the square minimizes the first eigenvalue among rectangles under area normalization, when the Robin parameter $α\in \mathbb{R}$ is scaled by perimeter; that the square maximizes the second eigenvalue for a sharp range of $α$-values; that the line segment minimizes the Robin spectral gap under diameter normalization for each $α\in \mathbb{R}$; and the square maximizes the spectral ratio among rectangles when $α>0$. Further, the spectral gap of each rectangle is shown to be an increasing function of the Robin parameter, and the second eigenvalue is concave with respect to $α$. Lastly, the shape of a Robin rectangle can be heard from just its first two frequencies, except in the Neumann case.

math.SP↗
arXiv2019

From Steklov to Neumann and beyond, via Robin: the Szegő way

The second eigenvalue of the Robin Laplacian is shown to be maximal for the disk among simply-connected planar domains of fixed area when the Robin parameter is scaled by perimeter in the form $α/L(Ω)$, and $α$ lies between $-2π$ and $2π$. Corollaries include Szegő's sharp upper bound on the second eigenvalue of the Neumann Laplacian under area normalization, and Weinstock's inequality for the first nonzero Steklov eigenvalue for simply-connected domains of given perimeter. The first Robin eigenvalue is maximal, under the same conditions, for the degenerate rectangle. When area normalization on the domain is changed to conformal mapping normalization and the Robin parameter is positive, the maximiser of the first eigenvalue changes back to the disk.

math.SP↗