Sergei Aleksandrovich Nazarov

Enforced stability of an eigenvalue in the continuous spectrum of a waveguide with an obstacle

Perturbations of an eigenvalue in the continuous spectrum of the Neumann problem for the Laplacian in a strip waveguide with an obstacle symmetric about the midline are studied. Such an eigenvalue is known to be unstable, and an arbitrarily small perturbation can cause it to leave the spectrum to become a complex resonance point. Conditions on the perturbation of the obstacle boundary are found under which the eigenvalue persists in the continuous spectrum. The result is obtained via the asymptotic analysis of an auxiliary object, namely, an augmented scattering matrix.

Artificial boundary conditions for finding surface waves in the problem of diffraction by a periodic boundary

Transparent artificial boundary conditions and an algorithm for computing the augmented scattering matrix are proposed for finding surface waves in a prescribed range of decay rates. An infinite-dimensional fictitious scattering operator is constructed that determines all waves decaying exponentially with distance from a periodic obstacle.

Trapped modes in a cylindrical elastic waveguide with a damping gasket

An infinite cylindrical body containing a three-dimensional heavy rigid inclusion with a sharp edge is considered. Under certain constraints on the symmetry of the body, it is shown that any prescribed number of eigenvalues of the elasticity operator can be placed on an arbitrary real interval (0, l) by choosing suitable physical properties of the inclusion. In the continuous spectrum, these points correspond to trapped modes, i.e., to exponentially decaying solutions to the homogeneous problem. The results can be used to design filters and dampers of elastic waves in a cylinder.

Bounded solutions in a T-shaped waveguide and the spectral properties of the Dirichlet ladder

The Dirichlet problem is considered on the junction of thin quantum waveguides (of thickness h ≪ 1) in the shape of an infinite two-dimensional ladder. Passage to the limit as h → +0 is discussed. It is shown that the asymptotically correct transmission conditions at nodes of the corresponding one-dimensional quantum graph are Dirichlet conditions rather than the conventional Kirchhoff transmission conditions. The result is obtained by analyzing bounded solutions of a problem in the T-shaped waveguide that the boundary layer phenomenon.

Calculation of characteristics of trapped modes in T-shaped waveguides

The spectrum of the Dirichlet problem for the Laplace operator in a plane T-shaped waveguide is investigated. The critical width of the half-strip branch is determined such that, if the width is greater, the waveguide has no discrete spectrum. The existence of a critical width is proved theoretically.

Non-scattering wavenumbers and far field invisibility for a finite set of incident/scattering directions

We investigate a time harmonic acoustic scattering problem by a penetrable inclusion with compact support embedded in the free space. We consider cases where an observer can produce incident plane waves and measure the far field pattern of the resulting scattered field only in a finite set of directions. In this context, we say that a wavenumber is a non-scattering wavenumber if the associated relative scattering matrix has a non trivial kernel. Under certain assumptions on the physical coefficients of the inclusion, we show that the non-scattering wavenumbers form a (possibly empty) discrete set. Then, in a second step, for a given real wavenumber and a given domain D, we present a constructive technique to prove that there exist inclusions supported in D for which the corresponding relative scattering matrix is null. These inclusions have the important property to be impossible to detect from far field measurements. The approach leads to a numerical algorithm which is described at the end of the paper and which allows to provide examples of (approximated) invisible inclusions.

Scalar boundary value problems on junctions of thin rods and plates. I. Asymptotic analysis and error estimates

We derive asymptotic formulas for the solutions of the mixed boundary value problem for the Poisson equation on the union of a thin cylindrical plate and several thin cylindrical rods. One of the ends of each rod is set into a hole in the plate and the other one is supplied with the Dirichlet condition. The Neumann conditions are imposed on the whole remaining part of the boundary. Elements of the junction are assumed to have contrasting properties so that the small parameter, i.e. the relative thickness, appears in the differential equation, too, while the asymptotic structures crucially depend on the contrastness ratio. Asymptotic error estimates are derived in anisotropic weighted Sobolev norms.

Asymptotics of eigenvalues of the Dirichlet problem in a skewed ℐ-shaped waveguide

Asymptotics are constructed and justified for the eigenvalues of the Dirichlet problem for the Laplacian in a waveguide consisting of a unit strip and a semi-infinite strip joined at a small angle ɛ ∈ (0, π/2). Some properties of the discrete spectrum are established, and open questions are stated.

Asymptotic behavior of spectral gaps in a regularly perturbed periodic waveguide

Since the spectrum of a periodic waveguide is the union of a countable family of closed bounded segments (spectral bands), it can contain opened spectral gaps, i.e., intervals in the real positive semi-axis that are free of the spectrum but have both endpoints in it. A cylindrical waveguide has an intact spectrum that is a closed ray. We consider a small periodic perturbation of the waveguide wall, and, by means of an asymptotic analysis of the eigenvalues in the model problem on the periodicity cell, we show how a spectral gap opens when the cylindrical waveguide converts into a periodic one. Indeed, a cylindrical waveguide can be interpreted as a periodic one with an arbitrary period, but all its spectral bands touch each other. A periodic perturbation of the waveguide wall provides the splitting of the band edges. This effect is known in the physical literature for waveguides of different shapes, and, in this paper, we provide a rigorous mathematical proof of the effect.Several variants of the edge splitting (alone and coupled, simple and multiple knots) are examined. Explicit formulas are obtained for a plane waveguide.

Surface enthalpy and elastic properties of blood vessels

1. Blood vessels form one of the most complicated and important functional systems of humans and other mammals subjected to many risks and poorly amenable to medicinal treatment. The mathematical simulation of blood flow through the aorta, arteries, veins, peripheral vessels, and capillaries both sepa rately and as a whole still remains an important problem. At the same time, the published bloodflow models give no possibility to take into account the complex (composite and anisotropic) structure of walls of vessels. Moreover, the formulation itself of experiments on determining elastic characteristics of walls of ves sels is at the stage of development (see [1, 2] and the review of literature in monograph [3, sect. 8]). This study is devoted to derivation of certain relations for deformable bloodvessel walls on the basis of the conventional representation of their laminar structure (see, for example, [1, 4, 3]). By means of the dimensionreduction procedure and on the basis of the concept of surface enthalpy, we derived a simple formula for the effective elasticmodulus tensor. Contrary to the habitual formulation of the mathematical problem, the blood vessel is not beforehand assumed as a circular cylinder, and the occurrence of a variable curvature, which does not complicate principally the asymptotic analysis, enables us to investigate the deformation of walls of arteries due to damage, such as inhomogeneous calcification (hyalinosis or calcinosis), oblong atherosclerotic deposits (cholesterolplaques), or various surgical treatments (clipping, stenting, and stitching).