Coercivity of Combined Boundary Integral Equations in High‐Frequency Scattering

We prove that the standard second‐kind integral equation formulation of the exterior Dirichlet problem for the Helmholtz equation is coercive (i.e., sign‐definite) for all smooth convex domains when the wavenumber k is sufficiently large. (This integral equation involves the so‐called combined potential, or combined field, operator.) This coercivity result yields k‐explicit error estimates when the integral equation is solved using the Galerkin method, regardless of the particular approximation space used (and thus these error estimates apply to several hybrid numerical‐asymptotic methods developed recently). Coercivity also gives k‐explicit bounds on the number of GMRES iterations needed to achieve a prescribed accuracy when the integral equation is solved using the Galerkin method with standard piecewise‐polynomial subspaces. The coercivity result is obtained by using identities for the Helmholtz equation originally introduced by Morawetz in her work on the local energy decay of solutions to the wave equation. © 2015 Wiley Periodicals, Inc.     

The quasihomogeneous function method and Fock's problem

The diffraction of a high-frequency wave by a smooth convex body near the tangency point of the limiting ray to the surface is restated as the scattering problem for the Schrodinger equation with a linear potential on a half-axis. Various prior estimates for the scattering problem are used in order to prove existence, uniqueness, and smoothness theorems. The corresponding solution satisfies the principle of limiting absorption. The formal solution of the corresponding Schrodinger equation in the form of quaslhomogeneous functions is essentially used in our constructions.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 148, pp. 144–151, 1985.     

High-frequency asymptotics of wave field diffracted by plane angular sector. I

The problem of plane-wave diffraction by an angular sector is examined. It is assumed that the wave process is described by the Helmholtz equation and that the Dirichlet or Neumann boundary conditions are satisfied on the sector. An exact solution of the problem is constructed in the form of a Sommerfeld integral, which is convenient for study of the problem in a high-frequency approximation.     

Scattering of High-Frequency Electromagnetic Waves by the Vertex of a Perfectly Conducting Cone (Singular Directions)

An analytic expression for the electromagnetic wave scattered in singular directions on the vertex of a perfectly conducting cone is obtained. The approach used in the paper is a generalization to the electromagnetic case of the approach previously developed by the authors. In singular directions, the spherical front set of a wave scattered by the vertex is tangent to the front set of a wave reflected by the cone surface. The wave field is expressed in terms of parabolic-cylinder functions. Bibliography: 8 titles.     

Scattering problem for the Schrödinger equation in the case of a potential linear in time and coordinate. I. Asymptotics in the shadow zone

The formal asymptotics of the scattering problem for the Schrödinger equation with a linear potential as x+¦t¦+ is studied. In the shadow zone a formal asymptotic expansion is constructed which is matched with the known asymptotics as t– The expansion constructed loses asymptotic character near the curve x=1/6 t³ (in the so-called projector zone). An assumption regarding the analogous behavior of the asymptotic series in the projector zone makes it possible to construct an expansion in the post-projection zone which goes over into the formulas for creeping waves.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 140, pp. 6–17, 1984.In conclusion, the authors would like to bring to the reader's attention another approach to asymptotics in the projector zone proposed by M. M. Popov (see the present collection).     

The scattering problem for the Schrödinger equation with a potential linear in time and in space. II. Correctness,smoothness, behavior of the solution at infinity

We study the scattering problem associated with the behavior of whispering gallery waves near the inflection point of the boundary. In order to solve the scattering problem, we prove the theorems of existence, uniqueness and smoothness of the solution. The formal asymptotic behavior is justified for t– and superexponential smallness of the wave field in the shadow zone is proved.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 148, pp. 13–29, 1985.     

Diffraction of plane waves by conical obstacles

The problem of plane wave incidence on a conical obstacle of arbitrary cross section is analyzed. Constructing a solution in the form of a Watson integral and its subsequent investigation allow one to describe a spherical wave scattered by the vertex of the cone. The general scheme is illustrated by examples of diffraction by circular and elliptic cones.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 173, pp. 142–154, 1988.     

A hybrid numerical-asymptotic boundary integral method for high-frequency acoustic scattering

We propose a new robust method for the computation of scattering of high-frequency acoustic plane waves by smooth convex objects in 2D. We formulate this problem by the direct boundary integral method, using the classical combined potential approach. By exploiting the known asymptotics of the solution, we devise particular expansions, valid in various zones of the boundary, which express the solution of the integral equation as a product of explicit oscillatory functions and more slowly varying unknown amplitudes. The amplitudes are approximated by polynomials (of minimum degree d) in each zone using a Galerkin scheme. We prove that the underlying bilinear form is continuous in L ₂, with a continuity constant that grows mildly in the wavenumber k. We also show that the bilinear form is uniformly L ₂-coercive, independent of k, for all k sufficiently large. (The latter result depends on rather delicate Fourier analysis and is restricted in 2D to circular domains, but it also applies to spheres in higher dimensions.) Using these results and the asymptotic expansion of the solution, we prove superalgebraic convergence of our numerical method as d → ∞ for fixed k. We also prove that, as k → ∞, d has to increase only very modestly to maintain a fixed error bound (d ∼ k ^1/9 is a typical behaviour). Numerical experiments show that the method suffers minimal loss of accuracy as k →∞, for a fixed number of degrees of freedom. Numerical solutions with a relative error of about 10⁻⁵ are obtained on domains of size for k up to 800 using about 60 degrees of freedom.     

Radiochemical characterization and decontamination of rare-earth-element concentrate recovered from uranium leach liquors

Journal of Radioanalytical and Nuclear Chemistry - The paper deals with two rare earth elements (REE) concentrates recovered from uranium leach liquors after sorption separation of uranium....