Mathematics

Existence of the eigenvalues of the discrete-time quantum walks is deeply related to localization of the walks. We revealed the distributions of the eigenvalues given by the splitted generating function method (the SGF method) of the quantum walks we had treated in our previous studies. In particular, we focused on two kinds of the Hadamard walk with one defect models and the two-phase QWs that have phases at the non-diagonal elements of the unitary transition operators. As a result, we clarified the characteristic parameter dependence for the distributions of the eigenvalues with the aid of numerical simulation.

In [Camano, Lackner, Monk, SIAM J. Math. Anal., Vol. 49, No. 6, pp. 4376-4401 (2017)] it was suggested to use Stekloff eigenvalues for Maxwell equations as target signature for nondestructive testing via inverse scattering. The authors recognized that in general the eigenvalues due not correspond to the spectrum of a compact operator and hence proposed a modified eigenvalue problem with the desired properties. The Fredholmness and the approximation of both problems were analyzed in [Halla, arXiv:1909.00689 (2019)]. The present work considers the original eigenvalue problem in the selfadjoint case. We report that apart for a countable set of particular frequencies, the spectrum consists of three disjoint parts: The essential spectrum consisting of the point zero, an infinite sequence of positive eigenvalues which accumulate only at infinity and an infinite sequence of negative eigenvalues which accumulate only at zero. The analysis is based on a representation of the operator as block operator. For small/big enough eigenvalue parameter the Schur-complements with respect to different components can be build. For each Schur-complement the existence of an infinite sequence of eigenvalues is proved via a fixed point technique similar to [Cakoni, Haddar, Applicable Analysis, 88:4, 475-493 (2009)]. The modified eigenvalue problem considered in the above references arises as limit of one of the Schur-complements.

We introduce the notion of Benjamini-Schramm convergence for quantum graphs. This notion of convergence, intended to play the role of the already existing notion for discrete graphs, means that the restriction of the quantum graph to a randomly chosen ball has a limiting distribution. We prove that any sequence of quantum graphs with uniformly bounded data has a convergent subsequence in this sense. We then consider the empirical spectral measure of a convergent sequence (with general boundary conditions and edge potentials) and show that it converges to the expected spectral measure of the limiting random rooted quantum graph. These results are similar to the discrete case, but the proofs are significantly different.

Consider (M,g) a compact, boundaryless Riemannian manifold admitting an Anosov geodesic flow. Let ϵ<min{1, λ max 2 } where λ max is the maximal expansion rate for (M,g) . We study the semiclassical measures μ sc of ϵ -logarithmic modes, which are quasimodes spectrally supported in intervals of width ϵ ??|log?�| , of the Laplace-Beltrami operator on M . Under a technical assumption on the support of a signed measure related to μ sc , we show that the lower bound for the Kolmogorov-Sinai entropy of μ sc generalizes that of Ananthamaran-Koch-Nonnenmacher. A feature of our results is the quantitative nature of our bounds on hyperbolic manifolds. Via previous results of the author with Nonnenmacher and Silberman, we provide another demonstration of the existence of ϵ -logarithmic modes whose semiclassical measures possess zero entropy ergodic components yet have other components with positive entropy.

We develop the basic theory of ergodic Schrödinger operators, which is well known for ergodic probability measures, in the case of a base dynamics on an infinite measure space. This includes the almost sure constancy of the spectrum and the spectral type, the definition and discussion of the density of states measure and the Lyapunov exponent, as well as a version of the Pastur--Ishii theorem. We also give some counterexamples that demonstrate that some results do not extend from the finite measure case to the infinite measure case. These examples are based on some constructions in infinite ergodic theory that may be of independent interest.

We introduce concepts of essential numerical range for the linear operator pencil λ↦A−λB . In contrast to the operator essential numerical range, the pencil essential numerical ranges are, in general, neither convex nor even connected. The new concepts allow us to describe the set of spectral pollution when approximating the operator pencil by projection and truncation methods. Moreover, by transforming the operator eigenvalue problem Tx=λx into the pencil problem BTx=λBx for suitable choices of B , we can obtain non-convex spectral enclosures for T and, in the study of truncation and projection methods, confine spectral pollution to smaller sets than with hitherto known concepts. We apply the results to various block operator matrices. In particular, Theorem 4.12 presents substantial improvements over previously known results for Dirac operators while Theorem 4.5 excludes spectral pollution for a class of non-selfadjoint Schrödinger operators which it has not been possible to treat with existing methods.

We discuss connections between the essential self-adjointness of a symmetric operator and the constancy of functions which are in the kernel of the adjoint of the operator. We then illustrate this relationship in the case of Laplacians on both manifolds and graphs. Furthermore, we discuss the Green's function and when it gives a non-constant harmonic function which is square integrable.

Based on the notion of the resolvent and on the Hilbert identities, this paper presents a number of classical results in the theory of differential operators and some of their applications to the theory of automorphic functions and number theory from a unified point of view. For instance, for the Sturm-Liouville operator there is a derivation of the Gelfand-Levitan trace formula, and for the one-dimensional Schroedinger operator a derivation of Faddeev's formula for the characteristic determinant and the Zakharov-Faddeev trace identities. Recent results on the spectral theory of a certain functional-difference operator arising in conformal field theory are then presented. The last section of the survey is devoted to the Laplace operator on a fundamental domain of a Fuchsian group of the first kind on the Lobachevsky plane. An algebraic scheme is given for proving analytic continuation of the integral kernel of the resolvent of the Laplace operator and the Eisenstein-Maass series. In conclusion, there is a discussion of the relation between the values of the Eisenstein-Maass series at Heegner points and Dedekind zeta-functions of imaginary quadratic fields, and it is explained why pseudo-cuspforms for the case of the modular group do not provide any information about the zeros of the Riemann zeta-function.

We describe the Dirichlet spectrum structure for the Fichera layers and crosses in any dimension n≥3 . Also the application of the obtained results to the classical Brownian exit times problem in these domains.

We derive explicit Krein resolvent identities for generally singular Sturm-Liouville operators in terms of boundary condition bases and the Lagrange bracket. As an application of the resolvent identities obtained, we compute the trace of the resolvent difference of a pair of self-adjoint realizations of the Bessel expression − d 2 /d x 2 +( ν 2 −(1/4)) x −2 on (0,∞) for values of the parameter ν∈[0,1) and use the resulting trace formula to explicitly determine the spectral shift function for the pair.