Function Spaces Associated with Schrödinger Operators: The Pöschl-Teller Potential

We address the function space theory associated with the Schrödinger operator H = ?d²/dx² + V. The discussion is featured with potential V (x) = ?n(n + 1) sech²x, which is called in quantum physics Pöschl-Teller potential. Using a dyadic system, we introduce Triebel-Lizorkin spaces and Besov spaces associated with H. We then use interpolation method to identify these spaces with the classical ones for a certain range of p, q > 1. A physical implication is that the corresponding wave function ψ(t, x) = e^?itHf(x) admits appropriate time decay in the Besov space scale.     

The convergence of multidimensional fourier-bessel series

We study the convergence of series of eigenfunctions of the Laplacian in the unit ballB ^d. The problem is posed in the spacesL _rad ^p (L _ang ² ). A convergence result is obtained in the sharp range2d/(d + 1) <p <2d/(d-1). There is a close connection with the spherical summation of classical trigonometric expansions. The proofs involve weighted inequalities for singular integrals, as well as a precise decomposition of oscillatory integrals using van der Corput’s method.     

An inverse scattering problem for a perturbed Hill's operator

We consider the inverse scattering problem for the operator L=?d²/dx²+p(x)+q(x), x ∈ R¹. The perturbation potential q is expressed in terms of the periodic potential p and the scattering data. We also obtain identities for the eigenfunctions of the unperturbed Hill's operator L₀=?d²/dx²+p(x).     

Expansion in the eigenfunctions of the schrödinger equation with a potential having a periodic asymptotic behavior

A theorem is proved regarding the expansion in the eigenfunctions of the one-dimensional Schrödinger equationL = ?d ^z/dx ²+q(x)(?∞<x<∞)with a potential q(x), satisfying the condition $$\int\limits_0^{ + \infty } {(1 + x^2 )|q(x) - q_ \pm (x)|dx< \infty ,} $$ where q±(x) are periodic functions.     

Catalytic branching random walks in ℤ d with branching at the origin

A time-continuous branching random walk on the lattice ? ^d , d ≥ 1, is considered when the particles may produce offspring at the origin only. We assume that the underlying Markov random walk is homogeneous and symmetric, the process is initiated at moment t = 0 by a single particle located at the origin, and the average number of offspring produced at the origin is such that the corresponding branching random walk is critical. The asymptotic behavior of the survival probability of such a process at moment t → ∞ and the presence of at least one particle at the origin is studied. In addition, we obtain the asymptotic expansions for the expectation of the number of particles at the origin and prove Yaglom-type conditional limit theorems for the number of particles located at the origin and beyond at moment t.     

Über ein Turánsches Problem für ℓ-1 radiale, positiv definite Funktionen

Turán’s problem is to determine the greatest possible value of the integral ∫_? ^df(x)dx/ f (0) for positive definite functions f (x), x ∈ ?^d, supported in a given convex centrally symmetric body D ? ?^d. In this note we consider the 2-dimensional Turán problem for positive definite functions of the form f(x) = φ (∥x∥1), x ∈ ?², with φ supported in [0,π].     

On the nodal sets of toral eigenfunctions

We study the nodal sets of eigenfunctions of the Laplacian on the standard d-dimensional flat torus. The question we address is: Can a fixed hypersurface lie on the nodal sets of eigenfunctions with arbitrarily large eigenvalue? In dimension two, we show that this happens only for segments of closed geodesics. In higher dimensions, certain cylindrical sets do lie on nodal sets corresponding to arbitrarily large eigenvalues. Our main result is that this cannot happen for hypersurfaces with nonzero Gauss-Kronecker curvature. In dimension two, the result follows from a uniform lower bound for the L ²-norm of the restriction of eigenfunctions to the curve, proved in an earlier paper (Bourgain and Rudnick in C. R. Math. 347(21?C22):1249?C1253, 2009). In high dimensions we currently do not have this bound. Instead, we make use of the real-analytic nature of the flat torus to study variations on this bound for restrictions of eigenfunctions to suitable submanifolds in the complex domain. In all of our results, we need an arithmetic ingredient concerning the cluster structure of lattice points on the sphere. We also present an independent proof for the two-dimensional case relying on the ??abc-theorem?? in function fields.     

Multiple Phase Transitions in Long‐Range First‐Passage Percolation on Square Lattices

We consider a model of long‐range first‐passage percolation on the d‐dimensional square lattice ?d in which any two distinct vertices x,y ? ?^d are connected by an edge having exponentially distributed passage time with mean ‖ x – y ‖^α+o(1), where α > 0 is a fixed parameter and ‖·‖ is the l¹–norm on ?^d. We analyze the asymptotic growth rate of the set ß_t, which consists of all x ? ?^d such that the first‐passage time between the origin 0 and x is at most t as t → ∞. We show that depending on the values of α there are four growth regimes: (i) instantaneous growth for α < ^d, (ii) stretched exponential growth for α ? d,2d), (iii) superlinear growth for α ? (2d,2d + 1), and finally (iv) linear growth for α > 2d + 1 like the nearest‐neighbor first‐passage percolation model corresponding to α=∞. © 2015 Wiley Periodicals, Inc.     

Optimal configurations of lines and a statistical application

Motivated by the construction of confidence intervals in statistics, we study optimal configurations of 2 ^d ? 1 lines in real projective space ?? ^d?1. For small d, we determine line sets that numerically minimize a wide variety of potential functions among all configurations of 2 ^d ? 1 lines through the origin. Numerical experiments verify that our findings enable to assess efficiently the tightness of a bound arising from the statistical literature.     

Local Particles Numbers in Critical Branching Random Walk

Critical catalytic branching random walk on an integer lattice ? ^d is investigated for all d∈?. The branching may occur at the origin only and the start point is arbitrary. The asymptotic behavior, as time grows to infinity, is determined for the mean local particles numbers. The same problem is solved for the probability of the presence of particles at a fixed lattice point. Moreover, the Yaglom type limit theorem is established for the local number of particles. Our analysis involves construction of an auxiliary Bellman–Harris branching process with six types of particles. The proofs employ the asymptotic properties of the (improper) c.d.f. of hitting times with taboo. The latter notion was recently introduced by the author for a non-branching random walk on ? ^d .     

BOUNDEDNESS OF FRACTIONAL INTEGRALS IN HARDY SPACES WITH NON-DOUBLING MEASURE - In Memory of Professor Sun Yongsheng

Let μ be a Borel measure on Rd which may be non doubling. The only condition that μ must satisfy is μ(Q) ≤ col(Q)n for any cube Q () Rd with sides parallel to the coordinate axes and for some fixed n with 0 ＜ n ≤ d. The purpose of this paper is to obtain a boundedness property of fractional integrals in Hardy spaces H1 (μ).     

Decay of eigenfunctions of elliptic PDE's,I

We study exponential decay of eigenfunctions of self-adjoint higher order elliptic operators on R^d${\mathbb{R}}^d$. We show that the possible (global) critical decay rates are determined algebraically. In addition we show absence of super-exponentially decaying eigenfunctions and a refined exponential upper bound.     

Inverse Sturm–Liouville problems with the potential known on an interior subinterval

The uniqueness problem of inverse Sturm–Liouville problems with the potential known on an interior subinterval is considered. We prove that the potential on the entire interval and boundary conditions are uniquely determined in terms of the known eigenvalues and some information on the eigenfunctions at some interior point (interior spectral data). Moreover, we also concern with the situation where the potential q is C^2k-smoothness at some given points.     

Stability For a Multidimensional Inverse Spectral Theorem

We consider the eigenvalue problem in Ω

Where Ω is a bounded domain in R^d with smooth boundary,a nd q is a bounded, measurable function on Ω The eigenvalue problem has discrete spectrum; we denote by and a nondecreasing sequence of eigenvalue and corresponding (orthonormal) eigenfunctions. It is known ([N–S–U]) that knowledge of the eigenvalues and the boundary values of the normal derivatives of the corresponding eigenfunctions is sufficient to uniquely determine a coefficient, q.     

Fast Computation of High‐Frequency Dirichlet Eigenmodes via Spectral Flow of the Interior Neumann‐to‐Dirichlet Map

We present a new algorithm for numerical computation of large eigenvalues and associated eigenfunctions of the Dirichlet Laplacian in a smooth, star‐shaped domain in ?^d, d ≥ 2. Conventional boundary‐based methods require a root search in eigenfrequency k, hence take O(N³) effort per eigenpair found, where N = O(k^d?1) is the number of unknowns required to discretize the boundary. Our method is O(N) faster, achieved by linearizing with respect to k the spectrum of a weighted interior Neumann‐to‐Dirichlet (NtD) operator for the Helmholtz equation. Approximations to the square roots k_j of all O(N) eigenvalues lying in [k ? ?, k], where ? = O(1), are found with O(N³) effort. We prove an error estimate with C independent of k. We present a higher‐order variant with eigenvalue error scaling empirically as O(?⁵) and eigenfunction error as O(?³), the former improving upon the “scaling method” of Vergini and Saraceno. For planar domains (d = 2), with an assumption of absence of spectral concentration, we also prove rigorous error bounds that are close to those numerically observed. For d = 2 we compute robustly the spectrum of the NtD operator via potential theory, Nyström discretization, and the Cayley transform. At high frequencies (400 wavelengths across), with eigenfrequency relative error 10^?10, we show that the method is 10³ times faster than standard ones based upon a root search. © 2014 Wiley Periodicals, Inc.     

Multiple tunnel effect for dispersive waves on a star-shaped network: an explicit formula for the spectral representation

We consider the Klein?CGordon equation on a star-shaped network composed of n half-axes connected at their origins. We add a potential that is constant but different on each branch. The corresponding spatial operator is self-adjoint, and we state explicit expressions for its resolvent and its resolution of the identity in terms of generalized eigenfunctions. This leads to a generalized Fourier-type inversion formula in terms of an expansion in generalized eigenfunctions. Further, we prove the surjectivity of the associated transformation, thus showing that it is in fact a spectral representation. The characteristics of the problem are marked by the non-manifold character of the star-shaped domain. Therefore, the approach via the Sturm?CLiouville theory for systems is not well-suited. The considerable effort to construct explicit formulas involving the generalized eigenfunctions that incorporate the tunnel effect is justified for example by the perspective to study the influence of this effect on the L ^??-time decay of solutions.     

Convergence of Averages in the Ergodic Theorem for Groups ?d

Some estimates of the rates of convergence in the ergodic theorem for actions of groups ^d are given. Moreover, a martingale-ergodic theorem for ^d is proved. This theorem may be considered as an ergodic theorem in which the exact initial coordinates of points in the phase space are gradually forgotten. Bibliography: 9 titles.     

The extraordinary spectral properties of radially periodic Schrödinger operators

Since it became clear that the band structure of the spectrum of periodic Sturm-Liouville operatorst = - (d²/dr²) +q(r) does not survive a spherically symmetric extension to Schrödinger operatorsT =- Δ+ V with V(x) =q(¦x¦) for x ∈ ?^d,d ∈ ? 1, a wealth of detailed information about the spectrum of such operators has been acquired. The observation of eigenvalues embedded in the essential spectrum [μ₀, ∞[ ofT with exponentially decaying eigenfunctions provided evidence for the existence of intervals of dense point spectrum, eventually proved by spherical separation into perturbed Sturm-Liouville operatorst _c = t +(c/r ²). Subsequently, a numerical approach was employed to investigate the distribution of eigenvalues ofT more closely. An eigenvalue was discovered below the essential spectrum in the cased = 2, and it turned out that there are in fact infinitely many, accumulating at μ₀. Moreover, a method based on oscillation theory made it possible to count eigenvalues oft _c contributing to an interval of dense point spectrum ofT. We gained evidence that an asymptotic formula, valid forc → ∞, does in fact produce correct numbers even for small values of the coupling constant, such that a rather precise picture of the spectrum of radially periodic Schrödinger operators has now been obtained.     

Simulation of Stationary Gaussian Processes in [0, 1] d

Abstract

A method for simulating a stationary Gaussian process on a fine rectangular grid in [0, 1]^d ??^d is described. It is assumed that the process is stationary with respect to translations of ?^d, but the method does not require the process to be isotropic. As with some other approaches to this simulation problem, our procedure uses discrete Fourier methods and exploits the efficiency of the fast Fourier transform. However, the introduction of a novel feature leads to a procedure that is exact in principle when it can be applied. It is established that sufficient conditions for it to be possible to apply the procedure are (1) the covariance function is summable on ?^d, and (2) a certain spectral density on the d-dimensional torus, which is determined by the covariance function on ?^d, is strictly positive. The procedure can cope with more than 50,000 grid points in many cases, even on a relatively modest computer. An approximate procedure is also proposed to cover cases where it is not feasible to apply the procedure in its exact form.     

Minimal triangulations of Kummer varieties

By an (abstract) Kummer variety K^d we mean the d-dimensiona1 torus T^d modulo the involution ? ? — ?. The 2^d elements in T^d of order two are fixed points of the involution and therefore K^d has 2^d isolated singularities (for d ≧ 3). Any simplicial decomposition of K^d must have at least as many vertices. In this paper we describe a highly symmetrical simplicial decomposition of K^d with 2^d vertices such that the link of each vertex is a combinatorial real projective space ?P^d-1 with 2^d—1 vertices. The automorphism group of order (d + 1)! 2^d admits a natural representation in the affine group of dimension d over the field with two elements. A particular case is the classical Kummer surface with 16 nodes (d=4). In this case our 16-vertex triangulation has a close relationship with the abstract Kummer configuration 16₆.