arXiv subjects

Perry Kleinhenz

Publications and source records attributed to Perry Kleinhenz.

14 recordsLinked to original sources

arXiv2026

Hypoellipticity on time-periodic space-times

We study the hypoellipticity of operators on a product type Lorentzian manifold where the time variable is periodic. In particular, we prove that hypoellipticity holds for such time-periodic equations, with a stronger estimate when the mass parameter or time period lies outside a set of arbitrarily small measure. We consider both compact and noncompact spatial manifolds and provide explicit examples involving the wave operator. The proof relies on a Fourier series decomposition and asymptotics for eigenvalue counting functions.

math.AP↗

arXiv2026

Optimal decay for waves damped by superellipses

Energy decay rates for solutions of the damped wave equation on the torus are known to be influenced by the geometry of the damped set and the growth properties of the damping. In this paper we produce lower bounds on energy decay rates for a class of damping which are positive on a superellipse and grow polynomially like the distance to the boundary of the superellipse. The energy decay rates we obtain depend explicitly on the exponent used to define the superellipse and the polynomial power. We show these rates are sometimes optimal. The proof adapts quasimodes from $y$-invariant damping using a simplification of the usual normal form argument.

math.AP↗

arXiv2026

Observability of Schr\"odinger equations in Euclidean space

In this paper we introduce a new dynamical condition, the comb geometric control condition, which is sufficient for observability of the Schr\"odinger equation in Euclidean space. We provide examples which show this condition is strictly weaker than the observation set being open and periodic. We also prove for the fractional Schr\"odinger equation that for observation functions which are uniformly continuous, the geometric control condition is equivalent to observability and implies arbitrary time observability. The proofs rely on uncertainty principles for frequency localized functions which are proved using a semiclassical propagation of singularities approach.

math.AP↗

arXiv2025

Integrated Local Energy Decay for Waves with Time-Dependent Damping

We prove integrated local energy decay for solutions of the damped wave equation with time-dependent damping satisfying an appropriate generalization of the geometric control condition on asymptotically flat, stationary space-times. We first obtain a high frequency estimate, which we prove via a positive commutator estimate using an escape function explicitly constructed in terms of the damping around individual space-time trajectories. We combine the high frequency estimate with low and medium frequency results for the undamped problem, then we handle the damping term as a perturbation to obtain local energy decay.

math.AP↗

arXiv2025

Geometry of wave damping on the torus

Energy decay rates of damped waves on the torus depend on the behavior of the damping near the undamped region and on the geometry of the damped set. In this paper we refine these geometric considerations, by introducing the concept of order of a glancing undamped point, and estimating decay rates in terms of this order. The proof is based on generalizing an averaging argument due to Sun. We also show that damping sets which attain these improvements are generic among polygons and smooth curves.

math.AP↗

arXiv2025

Sharp energy decay rates for the damped wave equation on the torus via non-polynomial derivative bound conditions

For the damped wave equation on the torus, when some geodesics never meet the positive set of the damping, energy decay rates are known to depend on derivative bounds and growth properties of the damping near the boundary of its support, as well as the geometry of the support of the damping. In this paper we obtain, sometimes sharp, energy decay rates for damping which satisfy more general non-polynomial derivative bounds and growth properties. We also show how these rates can depend on the geometry of the support of the damping. We prove general results and apply them to examples of damping growing exponentially slowly, polynomial-logarithmically, or logarithmically. The decay rates found for these examples interpolate between known sharp rates for purely polynomial damping. The proof relies on estimating the solution at very fine, non-poynomial, semiclassical scales to obtain resolvent estimates, which are then converted to energy decay rates via semigroup theory.

math.AP↗

arXiv2023

Optimal backward uniqueness and polynomial stability of second order equations with unbounded damping

For general second order evolution equations, we prove an optimal condition on the degree of unboundedness of the damping, that rules out finite-time extinction. We show that control estimates give energy decay rates that explicitly depend on the degree of unboundedness, and establish a dilation method to turn existing control estimates for one propagator into those for another in the functional calculus. As corollaries, we prove Schr\"odinger observability gives decay for unbounded damping, weak monotonicity in damping, and quantitative unique continuation and optimal propagation for fractional Laplacians. As applications, we establish a variety of novel and explicit energy decay results to systems with unbounded damping, including singular damping, linearised gravity water waves and Euler--Bernoulli plates.

math.AP↗

arXiv2023

Sharp conditions for exponential and non-exponential uniform stabilization of the time-dependent damped wave equation

It is classical that uniform stabilization of solutions to the autonomous damped wave equation is equivalent to every geodesic meeting the positive set of the damping, which is called the geometric control condition. In this paper, it is shown that for time-dependent damping a generalization of the geometric control condition is equivalent to uniform stabilization at an exponential rate. Additionally, upper and lower bounds on non-exponential uniform stabilization rates are computed for damping which do not satisfy this geometric control condition. Decay rates are guaranteed via an observability argument, including a new uniform time-dependent observability inequality. Bounds on decay rates are proved via a Gaussian beam construction.

math.AP↗

arXiv2022

Sharp polynomial decay for polynomially singular damping on the torus

We study energy decay rates for the damped wave equation with unbounded damping, without the geometric control condition. Our main decay result is sharp polynomial energy decay for polynomially controlled singular damping on the torus. We also prove that for normally $L^p$-damping on compact manifolds, the Schr\"odinger observability gives $p$-dependent polynomial decay, and finite time extinction cannot occur. We show that polynomially controlled singular damping on the circle gives exponential decay.

math.AP↗

arXiv2022

Energy decay for the time dependent damped wave equation

Energy decay is established for the damped wave equation on compact Riemannian manifolds where the damping coefficient is allowed to depend on time. Using a time dependent observability inequality, it is shown that the energy of solutions decays at an exponential rate if the damping coefficient satisfies a time dependent analogue of the classical geometric control condition. Existing time dependent observability inequalities are improved by removing technical assumptions on the permitted initial data.

math.AP↗

arXiv2020

Sharp exponential decay rates for anisotropically damped waves

In this article, we study energy decay of the damped wave equation on compact Riemannian manifolds where the damping coefficient is anisotropic and modeled by a pseudodifferential operator of order zero. We prove that the energy of solutions decays at an exponential rate if and only if the damping coefficient satisfies an anisotropic analogue of the classical geometric control condition, along with a unique continuation hypothesis. Furthermore, we compute an explicit formula for the optimal decay rate in terms of the spectral abscissa and the long-time averages of the principal symbol of the damping over geodesics, in analogy to the work of Lebeau for the isotropic case. We also construct genuinely anisotropic dampings which satisfy our hypotheses on the flat torus.

math.AP↗

arXiv2019

Decay rates for the damped wave equation with finite regularity damping

Decay rates for the energy of solutions of the damped wave equation on the torus are studied. In particular, damping invariant in one direction and equal to a sum of squares of nonnegative functions with a particular number of derivatives of regularity is considered. For such damping energy decays at rate $1/t^{2/3}$. If additional regularity is assumed the decay rate improves. When such a damping is smooth the energy decays at $1/t^{4/5-\delta}$. The proof uses a positive commutator argument and relies on a pseudodifferential calculus for low regularity symbols.

math.AP↗

arXiv2019

Sharp polynomial decay rates for the damped wave equation with H\"older-like damping

We study decay rates for the energy of solutions of the damped wave equation on the torus. We consider dampings invariant in one direction and bounded above and below by multiples of $x^{\beta}$ near the boundary of the support and show decay at rate $1/t^{\frac{\beta+2}{\beta+3}}$. In the case where $W$ vanishes exactly like $x^{\beta}$ this result is optimal by work of the second author. The proof uses a version of the Morawetz multiplier method.

math.AP↗

arXiv2018

Stabilization rates for the damped wave equation with H\"older-regular damping

We study the decay rate of the energy of solutions to the damped wave equation in a setup where the geometric control condition is violated. We consider damping coefficients which are $0$ on a strip and vanish like polynomials, $x^{\beta}$. We prove that the semigroup cannot be stable at rate faster than $1/t^{(\beta+2)/(\beta+3)}$ by producing quasimodes of the associated stationary damped wave equation. We also prove that the semigroup is stable at rate at least as fast as $1/t^{(\beta+2)/(\beta+4)}$. These two results establish an explicit relation between the rate of vanishing of the damping and rate of decay of solutions. Our result partially generalizes a decay result of Nonnemacher in which the damping is an indicator function on a strip.

math.AP↗