arXiv subjects

T. J. Christiansen

Publications and source records attributed to T. J. Christiansen.

At least 19 recordsLinked to original sources

arXiv2025

Low energy resolvent asymptotics of the multipole Aharonov--Bohm Hamiltonian

We compute low energy asymptotics for the resolvent of the Aharonov--Bohm Hamiltonian with multiple poles for both integer and non-integer total fluxes. For integral total flux we reduce to prior results in black-box scattering while for non-integral total flux we build on the corresponding techniques using an appropriately chosen model resolvent. The resolvent expansion can be used to obtain long-time wave asymptotics for the Aharonov--Bohm Hamiltonian with multiple poles. An interesting phenomenon is that if the total flux is an integer then the scattering resembles even-dimensional Euclidean scattering, while if it is half an odd integer then it resembles odd-dimensional Euclidean scattering. The behavior for other values of total flux thus provides an `interpolation' between these.

math.AP↗

arXiv2025

Unconditional wave decay in dimension two

We extend Burq's logarithmic decay rate [Bur98] to general compactly supported scatterers in dimension two. The main novelty is using recent results on low-frequency expansions to remove the requirement that the spectrum be regular at zero. This allows us to include, among other examples, arbitrary smooth obstacles with variable boundary conditions.

math.AP↗

arXiv2025

Low energy resolvent expansions in dimension two

The behavior of the resolvent at low energies has implications for many kinds of asymptotics, including for the scattering matrix and phase, for the Dirichlet-to-Neumann map, and for wave evolution. In this paper we present a robust method, based in part on resolvent identity arguments following Vodev and boundary pairing arguments following Melrose, for deriving such expansions, and implement it in detail for compactly supported perturbations of the Laplacian on $\mathbb R^2$. We obtain precise results for general self-adjoint black box perturbations, in the sense of Sjöstrand--Zworski, and also for some non-self-adjoint ones. The most important terms are the most singular ones, and we compute them in detail, relating them to spaces of zero eigenvalues and resonances.

math.AP↗

arXiv2024

From resolvent expansions at zero to long time wave expansions

We prove a general abstract theorem deducing wave expansions as time goes to infinity from resolvent expansions as energy goes to zero, under an assumption of polynomial boundedness of the resolvent at high energy. We give applications to obstacle scattering, to Aharonov--Bohm Hamiltonians, to scattering in a sector, and to scattering by a compactly supported potential.

math.AP↗

arXiv2024

Persistence and disappearance of negative eigenvalues in dimension two

We compute asymptotics of eigenvalues approaching the bottom of the continuous spectrum, and associated resonances, for Schrödinger operators in dimension two. We distinguish persistent eigenvalues, which have associated resonances, from disappearing ones, which do not. We illustrate the significance of this distinction by computing corresponding scattering phase asymptotics and numerical Breit--Wigner peaks. We prove all of our results for circular wells, and extend some of them to more general problems using recent resolvent techniques.

math.SP↗

arXiv2023

Singularities and asymptotic distribution of resonances for Schrödinger operators in one dimension

We obtain new results about the high-energy distribution of resonances for the one-dimensional Schrödinger operator. Our primary result is an upper bound on the density of resonances above any logarithmic curve in terms of the singular support of the potential. We also prove results about the distribution of resonances in sectors away from the real axis, and construct a class of potentials producing multiple sequences of resonances along distinct logarithmic curves, explicitly calculating the asymptotic location of these resonances. The results are unified by the use of an integral representation of the reflection coefficients.

math-ph↗

arXiv2023

Low energy scattering asymptotics for planar obstacles

We compute low energy asymptotics for the resolvent of a planar obstacle, and deduce asymptotics for the corresponding scattering matrix, scattering phase, and exterior Dirichlet-to-Neumann operator. We use an identity of Vodev to relate the obstacle resolvent to the free resolvent and an identity of Petkov and Zworski to relate the scattering matrix to the resolvent. The leading singularities are given in terms of the obstacle's logarithmic capacity or Robin constant. We expect these results to hold for more general compactly supported perturbations of the Laplacian on $\mathbb R^2$, with the definition of the Robin constant suitably modified, under a generic assumption that the spectrum is regular at zero.

math.AP↗

arXiv2022

The semiclassical structure of the scattering matrix for a manifold with infinite cylindrical end

We study the microlocal properties of the scattering matrix associated to the semiclassical Schrödinger operator $P=h^2Δ_X+V$ on a Riemannian manifold with an infinite cylindrical end. The scattering matrix at $E=1$ is a linear operator $S=S_h$ defined on a Hilbert subspace of $L^2(Y)$ that parameterizes the continuous spectrum of $P$ at energy $1$. Here $Y$ is the cross section of the end of $X$, which is not necessarily connected. We show that, under certain assumptions, microlocally $S$ is a Fourier integral operator associated to the graph of the scattering map $κ:\mathcal{D}_κ\to T^*Y$, with $\mathcal{D}_κ\subset T^*Y$. The scattering map $κ$ and its domain $\mathcal{D}_κ$ are determined by the Hamilton flow of the principal symbol of $P$. As an application we prove that, under additional hypotheses on the scattering map, the eigenvalues of the associated unitary scattering matrix are equidistributed on the unit circle.

math.SP↗

arXiv2021

Resolvent estimates on asymptotically cylindrical manifolds and on the half line

Manifolds with infinite cylindrical ends have continuous spectrum of increasing multiplicity as energy grows, and in general embedded resonances (resonances on the real line, embedded in the continuous spectrum) and embedded eigenvalues can accumulate at infinity. However, we prove that if geodesic trapping is sufficiently mild, then the number of embedded resonances and eigenvalues is finite, and moreover the cutoff resolvent is uniformly bounded at high energies. We obtain as a corollary the existence of resonance free regions near the continuous spectrum. We also obtain improved estimates when the resolvent is cut off away from part of the trapping, and along the way we prove some resolvent estimates for repulsive potentials on the half line which may be of independent interest.

math.AP↗

arXiv2021

Resolvent estimates, wave decay, and resonance-free regions for star-shaped waveguides

Using coordinates $(x,y)\in \mathbb R\times \mathbb R^{d-1}$, we introduce the notion that an unbounded domain in $\mathbb R^d$ is star shaped with respect to $x=\pm \infty$. For such domains, we prove estimates on the resolvent of the Dirichlet Laplacian near the continuous spectrum. When the domain has infinite cylindrical ends, this has consequences for wave decay and resonance-free regions. Our results also cover examples beyond the star-shaped case, including scattering by a strictly convex obstacle inside a straight planar waveguide.

math.AP↗

arXiv2021

Manifolds with cylindrical ends having a finite and positive number of embedded eigenvalues

We construct a surface with a cylindrical end which has a finite number of Laplace eigenvalues embedded in its continuous spectrum. The surface is obtained by attaching a cylindrical end to a hyperbolic torus with a hole. To our knowledge, this is the first example of a manifold with a cylindrical end whose number of eigenvalues is known to be finite and nonzero. The construction can be varied to give examples with arbitrary genus and with an arbitrarily large finite number of eigenvalues. The constructed surfaces also have resonance-free regions near the continuous spectrum and long-time asymptotic expansions of solutions to the wave equation.

math.AP↗

arXiv2021

Resonances for Schrödinger operators on infinite cylinders and other products

We study the resonances of Schrödinger operators on the infinite product $X=\mathbb{R}^d\times \mathbb{S}^1$, where $d$ is odd, $\mathbb{S}^1$ is the unit circle, and the potential $V\in L^\infty_c(X)$. This paper shows that at high energy, resonances of the Schrödinger operator $-Δ+V$ on $X=\mathbb{R}^d\times \mathbb{S}^1$ which are near the continuous spectrum are approximated by the resonances of $-Δ+V_0$ on $X$, where the potential $V_0$ given by averaging $V$ over the unit circle. These resonances are, in turn, given in terms of the resonances of a Schrödinger operator on $\mathbb{R}^d$ which lie in a bounded set. If the potential is smooth, we obtain improved localization of the resonances, particularly in the case of simple, rank one poles of the corresponding scattering resolvent on $\mathbb{R}^d$. In that case, we obtain the leading order correction for the location of the corresponding high energy resonances. In addition to direct results about the location of resonances, we show that at high energies away from the resonances, the resolvent of the model operator $-Δ+V_0$ on $X$ approximates that of $-Δ+V$ on $X$. If $d=1$, in certain cases this implies the existence of an asymptotic expansion of solutions of the wave equation. Again for the special case of $d=1$, we obtain a resonant rigidity type result for the zero potential among all real-valued potentials.

math.SP↗

arXiv2020

Wave asymptotics for waveguides and manifolds with infinite cylindrical ends

We describe wave decay rates associated to embedded resonances and spectral thresholds for waveguides and manifolds with infinite cylindrical ends. We show that if the cut-off resolvent is polynomially bounded at high energies, as is the case in certain favorable geometries, then there is an associated asymptotic expansion, up to a $O(t^{-k_0})$ remainder, of solutions of the wave equation on compact sets as $t \to \infty$. In the most general such case we have $k_0=1$, and under an additional assumption on the infinite ends we have $k_0 = \infty$. If we localize the solutions to the wave equation in frequency as well as in space, then our results hold for quite general waveguides and manifolds with infinite cylindrical ends. To treat problems with and without boundary in a unified way, we introduce a black box framework analogous to the Euclidean one of Sjöstrand and Zworski. We study the resolvent, generalized eigenfunctions, spectral measure, and spectral thresholds in this framework, providing a new approach to some mostly well-known results in the scattering theory of manifolds with cylindrical ends.

math.AP↗

arXiv2017

Resonant rigidity for Schrödinger operators in even dimensions

This paper studies the resonances of Schrödinger operators with bounded, compactly supported, real-valued potentials on d-dimensional Euclidean space, where d is even. If the potential V is non-trivial and d is not 4 then the meromorphic continuation of the resolvent of the Schrödinger operator has infinitely many poles, with a quantitative lower bound on their density. A somewhat weaker statement holds if d =4. We prove several inverse-type results. If the meromorphic continuations of the resolvents of two Schrödinger operators $-Δ+V_1$ and $-Δ+V_2$ have the same poles, with both potentials bounded, compactly supported and real-valued, if k is a natural number and if $V_1\in H^k({\mathbb R}^d; {\mathbb R})$, then $V_2\in H^k$ as well. Moreover, we prove that certain sets of isoresonant potentials are compact. We also show that the poles of the resolvent for a smooth potential determine the heat coefficients and that the (resolvent) resonance sets of two bounded, real-valued potentials with compact support cannot differ by a nonzero finite number of elements away from $0$.

math.SP↗

arXiv2015

A sharp lower bound for a resonance-counting function in even dimensions

This paper proves sharp lower bounds on a resonance counting function for obstacle scattering in even-dimensional Euclidean space without a need for trapping assumptions. Similar lower bounds are proved for some other compactly supported perturbations of the Laplacian on even-dimensional Euclidean space, for example, for the Laplacian for certain metric perturbations. The proof uses a Poisson formula for resonances, complementary to one proved by Zworski in even dimensions.

math.SP↗

arXiv2014

Lower bounds for resonance counting functions for obstacle scattering in even dimensions

In even dimensional Euclidean scattering, the resonances lie on the logarithmic cover of the complex plane. This paper studies resonances for obstacle scattering in ${\mathbb R}^d$ with Dirchlet or admissable Robin boundary conditions, when $d$ is even. Set $n_m(r)$ to be the number of resonances with norm at most $r$ and argument between $mπ$ and $(m+1)π$. Then $\lim\sup _{r\rightarrow \infty}\frac{\log n_m(r)}{\log r}=d$ if $m\in {\mathbb Z}\setminus \{ 0\}$.

math-ph↗

arXiv2013

Lower bounds for resonance counting functions for Schrödinger operators with fixed sign potentials in even dimensions

If the dimension $d$ is even, the resonances of the Schrödinger operator $-Δ+V$ on ${\mathbb R}^d$ with $V$ bounded and compactly supported are points on $Λ$, the logarithmic cover of ${\mathbb C} \setminus \{0\}$. We show that for fixed sign potentials $V$ and for nonzero integers $m$, the resonance counting function for the $m$th sheet of $Λ$ has maximal order of growth.

math.SP↗

arXiv2013

Some remarks on resonances in even-dimensional Euclidean scattering

The purpose of this paper is to prove some results about quantum mechanical black box scattering in even dimensions $d \geq 2$. We study the scattering matrix and prove some identities which hold for its meromorphic continuation onto $Λ$, the Riemann surface of the logarithm function. We relate the multiplicities of the poles of the continued scattering matrix to the multiplicities of the poles of the resolvent. Moreover, we show that the poles of the scattering matrix on the $m$th sheet of $Λ$ are related to the zeros of a scalar function defined on the physical sheet. This paper contains a number of results about "pure imaginary" resonances. As an example, in contrast with the odd-dimensional case, we show that in even dimensions there are no "purely imaginary" resonances on any sheet of $Λ$ for Schrödinger operators with potentials $0 \leq V \in L_0^\infty (\R^d)$.

math-ph↗