arXiv subjects

Kenichi Ito

Publications and source records attributed to Kenichi Ito.

15 recordsLinked to original sources

arXiv2026

Exponential growth and decay estimates for eigenfunctions with complex eigenvalues

We prove a priori exponential growth and decay estimates for locally square integrable eigenfunctions with complex eigenvalues of the non-self-adjoint Schr\"odinger operator. There appear two natural critical exponents, given by the imaginary parts of square roots of twice an eigenvalue. Any associated eigenfunction has the same exponential growth or decay rate as one of these, or greater than the upper one. The results may be seen as extensions of the Phragm\'en--Lindel\"of theorem, the Agmon estimates and Rellich's theorem to complex eigenvalues. For the proofs we adapt a commutator scheme of Ito--Skibsted~(2020) to the non-self-adjoint framework, presenting a unified approach to all of them. In order to control contributions from the imaginary parts of a potential and an eigenvalue, an additional commutator estimate is employed.

math.AP↗

arXiv2026

Characterization of spacetime singularities for the Schr\"odinger equation by initial state

We discuss spacetime singularities of a solution to the Schr\"odinger equation with a metric perturbation and a sublinear potential. The quasi-homogeneous wave front set, due to Lascar (1977), of a solution is characterized by that of the free solution, and a classical high-energy scattering data. In the one-dimensional case, it further reduces to the homogeneous wave front set, due to Nakamura (2005), of the initial time-slice. For the proof of the former result we implement an idea inspired by Nakamura (2009), which was originally devised for spatial singularities of the Schr\"odinger equation. As for the latter result, we use an exact Egorov-type formula for the free propagator, and a special partition of unity conforming with the classical flow.

math.AP↗

arXiv2026

Low energy resolvent estimates for slowly decaying attractive potentials

We discuss the low energy resolvent estimates for the Schr\"odinger operator with slowly decaying attractive potential. The main results are Rellich's theorem, the limiting absorption principle and Sommerfeld's uniqueness theorem. For the proofs we employ an elementary commutator method due to Ito--Skibsted, for which neither of microlocal or functional-analytic techniques is required.

math-ph↗

arXiv2025

Propagation of space-time singularities for perturbed harmonic oscillators

We discuss propagation of space-time singularities for the quantum harmonic oscillator with time-dependent metric and potential perturbations. Reformulating the quasi-homogeneous wave front set according to Lascar (1977) in a semiclassical manner, we obtain a characterization of its appearance in comparison with the unperturbed system. The idea of our proof is based on the argument of Nakamura (2009), which was originally devised for the analysis of spatial singularities of the Schr\"odinger equation, however, the application is non-trivial since the time is no more a parameter, but takes a part in the base variables.

math.AP↗

arXiv2025

Elementary commutator method for the Dirac equation with long-range perturbations

We present direct and elementary commutator techniques for the Dirac equation with long-range electric and mass perturbations. The main results are absence of generalized eigenfunctions and locally uniform resolvent estimates, both in terms of the optimal Besov-type spaces. With an additional massless assumption, we also obtain an algebraic radiation condition of projection type. For their proofs, following the scheme of Ito-Skibsted, we adopt, along with various weight functions, the generator of radial translations as conjugate operator, and avoid any of advanced functional analysis, pseudodifferential calculus, or even reduction to the Schr\"odinger equation. The results of the paper would serve as a foundation for the stationary scattering theory of the Dirac operator.

math-ph↗

arXiv2020

Hypergeometric expression for the resolvent of the discrete Laplacian in low dimensions

We present an explicit formula for the resolvent of the discrete Laplacian on the square lattice, and compute its asymptotic expansions around thresholds in low dimensions. As a by-product we obtain a closed formula for the fundamental solution to the discrete Laplacian. For the proofs we express the resolvent in a general dimension in terms of the Appell--Lauricella hypergeometric function of type $C$ outside a disk encircling the spectrum. In low dimensions it reduces to a generalized hypergeometric function, for which certain transformation formulas are available for the desired expansions.

math-ph↗

arXiv2019

Branching form of the resolvent at threshold for multi-dimensional discrete Laplacians

We consider the discrete Laplacian on $\mathbb Z^d$, and compute asymptotic expansions of its resolvent around thresholds embedded in continuous spectrum as well as those at end points. We prove that the resolvent has a square-root branching if $d$ is odd, and a logarithm branching if $d$ is even, and, moreover, obtain explicit expressions for these branching parts involving the Lauricella hypergeometric function. In order to analyze a non-degenerate threshold of general form we use an elementary step-by-step expansion procedure, less dependent on special functions.

math-ph↗

arXiv2019

Time-dependent scattering theory on manifolds

Based on our previous study [IS3] on the stationary scattering theory for the Schrodinger operator on a manifold possessing an escape function we complete our investigation by doing the time-dependent counterpart. A particular class of examples are manifolds with Euclidean and/or hyperbolic ends, possibly with unbounded and non-smooth obstacles. As an application we resolve a conjecture of [HPW] on cross-ends transmissions in its natural and strong form within the time-dependent framework.

math.DG↗

arXiv2018

Resolvent expansion for the Schrödinger operator on a graph with infinite rays

We consider the Schrödinger operator on a combinatorial graph consisting of a finite graph and a finite number of discrete half-lines, all jointed together, and compute an asymptotic expansion of its resolvent around the threshold $0$. Precise expressions are obtained for the first few coefficients of the expansion in terms of the generalized eigenfunctions. This result justifies the classification of threshold types solely by growth properties of the generalized eigenfunctions. By choosing an appropriate free operator a priori possessing no zero eigenvalue or zero resonance we can simplify the expansion procedure as much as that on the single discrete half-line.

math.SP↗

arXiv2017

Resolvent expansions for the Schrödinger operator on the discrete half-line

Simplified models of transport in mesoscopic systems are often based on a small sample connected to a finite number of leads. The leads are often modelled using the Laplacian on the discrete half-line $\mathbb N$. Detailed studies of the transport near thresholds require detailed information on the resolvent of the Laplacian on the discrete half-line. This paper presents a complete study of threshold resonance states and resolvent expansions at a threshold for the Schrödinger operator on the discrete half-line $\mathbb N$ with a general boundary condition. A precise description of the expansion coefficients reveals their exact correspondence to the generalized eigenspaces, or the threshold types. The presentation of the paper is adapted from that of Ito-Jensen [Rev.\ Math.\ Phys.\ {\bf 27} (2015), 1550002 (45 pages)], implementing the expansion scheme of Jensen-Nenciu [Rev.\ Math.\ Phys.\ \textbf{13} (2001), 717--754, \textbf{16} (2004), 675--677] in its full generality.

math-ph↗

arXiv2013

A complete classification of threshold properties for one-dimensional discrete Schrödinger operators

We consider the discrete one-dimensional Schrödinger operator $H=H_0+V$, where $(H_0x)[n]=-(x[n+1]+x[n-1]-2x[n])$ and $V$ is a self-adjoint operator on $\ell^2(\mathbb{Z})$ with a decay property given by $V$ extending to a compact operator from $\ell^{\infty,-β}(\mathbb{Z})$ to $\ell^{1,β}(\mathbb{Z})$ for some $β\geq1$. We give a complete description of the solutions to $Hx=0$, and $Hx=4x$, $x\in\ell^{\infty,-β}(\mathbb{Z})$. Using this description we give asymptotic expansions of the resolvent of $H$ at the two thresholds $0$ and $4$. One of the main results is a precise correspondence between the solutions to $Hx=0$ and the leading coefficients in the asymptotic expansion of the resolvent around $0$. For the resolvent expansion we implement the expansion scheme of Jensen-Nenciu \cite{JN0, JN1} in the full generality.

math-ph↗

arXiv2012

Microlocal properties of scattering matrices for Schrödinger equations on scattering manifolds

Let $M$ be a scattering manifold, i.e., a Riemannian manifold with asymptotically conic structure, and let $H$ be a Schrödinger operator on $M$. We can construct a natural time-dependent scattering theory for $H$ with a suitable reference system, and the scattering matrix is defined accordingly. We here show the scattering matrices are Fourier integral operators associated to a canonical transform on the boundary manifold generated by the geodesic flow. In particular, we learn that the wave front sets are mapped according to the canonical transform. These results are generalizations of a theorem by Melrose and Zworski, but the framework and the proof are quite different. These results may be considered as generalizations or refinements of the classical off-diagonal smoothness of the scattering matrix for 2-body quantum scattering on Euclidean spaces.

math.AP↗

arXiv2009

Remarks on the Fundamental Solution to Schrödinger Equation with Variable Coefficients

We consider Schrödinger operators $H$ on $R^n$ with variable coefficients. Let $H_0=-\frac12\triangle$ be the free Schrödinger operator and we suppose $H$ is a "short-range" perturbation of $H_0$. Then, under the nontrapping condition, we show the time evolution operator: $e^{-itH}$ can be written as a product of the free evolution operator $e^{-itH_0}$ and a Fourier integral operator $W(t)$, which is associated to the canonical relation given by the classical mechanical scattering. We also prove a similar result for the wave operators. These results are analogous to results by Hassell and Wunsch, but the assumptions, the proof and the formulation of results are considerably different. The proof employs an Egorov-type theorem similar to those used in previous works by the authors combined with a Beals-type characterization of Fourier integral operators.

math.AP↗

arXiv2008

Time-dependent scattering theory for Schrödinger operators on scattering manifolds

We construct a time-dependent scattering theory for Schrödinger operators on a manifold $M$ with asymptotically conic structure. We use the two-space scattering theory formalism, and a reference operator on a space of the form $R\times \partial M$, where $\partial M$ is the boundary of $M$ at infinity. We prove the existence and the completeness of the wave operators, and show that our scattering matrix is equivalent to the absolute scattering matrix, which is defined in terms of the asymptotic expansion of generalized eigenfunctions. Our method is functional analytic, and we use no microlocal analysis in this paper.

math-ph↗

arXiv2007

Singularities of solutions to Schrodinger equation on scattering manifold

In this paper we study microlocal singularities of solutions to Schrodinger equations on scattering manifolds, i.e., noncompact Riemannian manifolds with asymptotically conic ends. We characterize the wave front set of the solutions in terms of the initial condition and the classical scattering maps under the nontrapping condition. Our result is closely related to a recent work by Hassell and Wunsch, though our model is more general and the method, which relies heavily on scattering theoretical ideas, is simple and quite different. In particular, we use Egorov-type argument in the standard pseudodifferential symbol classes, and avoid using Legendre distributions. In the proof, we employ a microlocal smoothing property in terms of the radially homogenous wave front set, which is more precise than the preceding results.

math.AP↗