N-group neutron transport theory: A criticality problem in slab geometry

The steady-state equation for N-group neutron transport in slab geometry is written as an integral equation. A spectral analysis is made of the integral operator and related to the criticality problem. The method depends on a representation for the resolvent kernel for a subcritical slab and on analytic continuation in a complex parameter to characterize eigenvalues in terms of singularities of the resolvent. The analytic continuation is based on a bifurcation analysis of some nonlinear matrix integral equations whose solutions provide a matrix Wiener-Hopf factorization of the Fourier transform of the kernel of the transport operator.     

Higher-order quadratures for circulant preconditioned Wiener-Hopf equations

In this paper, we consider solving matrix systems arising from the discretization of Wiener-Hopf equations by preconditioned conjugate gradient (PCG) methods. Circulant integral operators as preconditioners have been proposed and studied. However, the discretization of these preconditioned equations by employing higher-order quadratures leads to matrix systems that cannot be solved efficiently by using fast Fourier transforms (FFTs). The aim of this paper is to propose new preconditioners for Wiener-Hopf equations. The discretization of these preconditioned operator equations by higher-order quadratures leads to matrix systems that involve only Toeplitz, circulant and diagonal matrix-vector multiplications and hence can be computed efficiently by FFTs in each iteration. We show that with the proper choice of kernel functions of Wiener-Hopf equations, the resulting preconditioned operators will have clustered spectra and therefore the PCG method converges very fast. Numerical examples are given to illustrate the fast convergence of the method and the improvement of the accuracy of the computed solutions with using higher-order quadratures.Research supported by the Cooperative Research Centre for Advanced Computational Systems.Research supported in part by Lee Ka Shing scholarship.     

Theory of a class of Wiener-Hopf equations of the first kind: Application to the Sommerfeld problem

A new theory of a class of Wiener-Hopf equations of the first kind in a space of distributions is presented. It is shown that the corresponding Wiener-Hopf operator is a Fredholm operator. This result is obtained by an appropriate modification of the standard Wiener-Hopf technique used for equations of the second kind. The nullity and defect numbers of the operator are determined from a factorization of the symbol. An application to the Sommerfeld problem is briefly considered.     

Fredholm theory of Wiener-Hopf equations in terms of realization of their symbols

The Fredholm properties (index, kernel, image, etc.) of Wiener-Hopf integral operators are described in terms of realization of the symbol for a class of matrix symbols that are analytic on the real line but not at infinity. The realizations are given in terms of exponentially dichotomous operators. The results obtained give a complete analogue of the earlier results for rational symbols.     

About one system of integral equations in kinetic theory

One system of integral convolution equations is considered on a half-line with an noninvertible matrix integral operator whose symbol has a fourth-order zero. The application of a special factorization method makes it possible to distinguish noninvertible factors in the original noninvertible operator and reduce the system to a new system with a nonsingular integral operator. The structural theorem of the existence of a solution to the original system is proved.     

GENUINE-OPTIMAL CIRCULANT PRECONDITIONERS FOR WIENER-HOPF EQUATIONS

1. IntroductionWienerHopf equations are integral equations defined on the haif line:where rr > 0, a(.) C L1(ro and g(.) E L2(at). Here R = (--oo,oo) and ty [0,oo). Inou-r discussions, we assume that a(.) is colljugate symmetric, i.e. a(--t) = a(t). WienerHop f equations arise in a variety of practical aPplicatiolls in mathematics and ellgineering, forinstance, in the linear prediction problems fOr stationary stochastic processes [8, pp.145--146],diffuSion problems and scattering problems […     

On a class of Hankel operators: Fredholm properties and invertibility

A study is presented on the Fredholm properties and invertibility of a Hankel integral operator inL ₊ ² () with a kernel function inS whose Fourier transformK is a measurable essentially bounded function in . This study is based on the properties of a Wiener-Hopf operator with a matrix valued symbol naturally associated with the operator mentioned above. Further results are obtained for the case whereK PC(), and an application to a diffraction problem is presented.     

Wiener-Hopf equation: Kernels representable as a superposition of complex-valued exponents

The paper studies the Wiener-Hopf equations with kernels representable as superposition of complex-valued exponents. Such kernels arise in the kinetic gas theory, in the radiation transfer, etc. By application of a special, three-factor expansion of the initial uninvertible operator, the solution of the considered equation is reduced to those of two simple Volterra equations and a Wiener-Hopf integral equation with a contractive operator. A structural existence theorem is proved.     

A fast realization of preconditioned Conjugate Gradients for Wiener-Hopf integral equations

We present an efficient implementation of the Conjugate Gradients algorithm for Wiener-Hopf integral equations based on finite rank approximations of the integral operator and the corresponding preconditioner. The resulting algorithm is of linear complexity. Numerical experiments with this implementation of the preconditioned Conjugate Gradients algorithm show significant speed-up in the ill-conditioned case. This algorithm acts on ill-conditioned equations as a regularization algorithm.     

On invertibility of matrix wiener-hopf operator on discrete linearly ordered Abelian group

A connection between an invertibility of a matrix Wiener-Hopf operator on a discrete linearly ordered Abelian group and a canonical factoribility of the matrix symbol of the operator is studied. A method of the paper [1] is extended to the case of the group . Necessary and sufficient conditions for a normal solvability, a generalized invertibility, and an invertibility of the operator with a strictly nonsingular 2×2 matrix symbol of a special kind are found. We also give necessary conditions of the factoribility and necessary and sufficient conditions of the canonical factoribility of this matrix symbol.     

A boundary integral equation method for three-dimensional crack problems in elasticity

This paper presents a solution procedure for three-dimensional crack problems via first kind boundary integral equations on the crack surface. The Dirichlet (Neumann) problem is reduced to a system of integral equations for the jump of the traction (of the field) across the crack surface. The calculus of pseudodifferential operators is used to derive existence and regularity of the solutions of the integral equations. With the concept of the principal symbol and the Wiener-Hopf technique we derive the explicit behavior of the densities of the integral equations near the edge of the crack surface. Based on the detailed regularity results we show how to improve the boundary element Galerkin method for our integral equations. Quasi-optimal asymptotic estimates for the Galerkin error are given.     

Singular Integral Operators with Cauchy Kernel in Fractional Spaces

We single out the Besov spaces that embed into the class of continuous functions and enjoy the Fredholm theory of linear singular integral equations with Cauchy kernel. We give basic results of this theory in the class of continuous (rather than Holder continuous) functions in terms of Besov spaces. Alongside elliptic operators we consider violations of ellipticity: the degeneration of the symbol of an operator at finitely many points.     

Wiener-Hopf operators on a subsemigroup of a discrete torsion free abelian group

In the paper Wiener-Hopf operators on a semigroup of nonnegative elements of a linearly quasi-ordered torsion free Abelian group are considered. Wiener-Hopf factorization of an invertible element of the group algebra is constructed, notions of a topological index and a factor index are introduced. It turns out that the set of factor indices for invertible elements of the group algebra is a linearly ordered group. It is shown that Wiener-Hopf operator with an invertible symbol is an one-side invertible operator and its invertibility properties are defined by the sign of the factor index of its symbol. Groups on which there exist nontrivial Fredholm Wiener-Hopf operators are described. As an example, all linear quasi-orders on the group ⁿ are found and corresponding Wiener-Hopf operators are considered.     

白噪声分析中广义算子值函数的Bochner-Wick积分

白噪声广义算子在白噪声分析理论及其应用中起着十分重要的作用. 本文主要讨论了白噪声广义算子值函数的积分及相关问题. 主要工作有: 引入了广义算子值测度的概念, 分别讨论了这种测度在象征和算子p-范数意义下的变差及相互关系; 借助于广义算子的Wick积运算, 引入了广义算子值函数关于广义算子值测度的一种积分---Bochner-Wick积分, 讨论了这种积分的性质, 建立了相应的收敛定理并且展示了其在量子白噪声理论中的应用; 探讨了Bochner-Wick积分的Fubini定理及相关问题.     

Inversion of convolution equations on a finite interval and realization triples

This paper develops further the connections between convolution equations and realization triples. Here the emphasis is on equations on a finite interval. For each system of equations an operator (called indicator) is introduced which can be used to describe the inversion properties of the system. This indicator may be of simpler form than the convolution operator defined by the equations even for the case when the corresponding symbol is non-rational     

Trace theorems for pseudo-differential operators

An integral operator with smooth kernel can always be restricted to a hypersurfaceS. Acutally, it is again an integral operator and its kernel is the restriction (in both variables) of the original one toS. Here we study restrictions of pseudo-differential operators of arbitrary order. We find sufficient and (to some extent) necessary conditions on the symbol ensuring existence of the restriction. These conditions require the vanishing of some geometrical invariants defined on the conormal bundle of the hypersurface. In particular, for a pseudo-differential operator of orderm, the principal symbol should vanish of order [m]+2 and the subprincipal symbol of order [m]+1. These classical invariants are sufficient to treat the problem for the casem<1, but in the general case we need to introduce new higher order invariants related to the operator and the hypersurface.     

On some properties of systems of Volterra integral equations of the fourth kind with kernel of convolution type

We consider the system of integral equations of the form Ax +V x = Ψ, where V is the Volterra operator with kernel of convolution type and A is a constant matrix, det A = 0. We prove an existence theorem and establish necessary and sufficient conditions for the kernel of the operator of the system to be trivial.     

Optimal and Approximate Solutions of Singular Integral Equations by Means of Reproducing Kernels

A reproducing kernel method is proposed to obtain the optimal and approximate solutions of Carleman singular integral equations. Therefore, we will be mostly interested in singular integral equations with a Cauchy type kernel and whose coefficients are real or complex valued functions. The new method and corresponding concepts allow the analysis of associated discrete singular integral equations and corresponding inverse source problems in appropriate frameworks.     

(Modified) Fredholm Determinants for Operators with Matrix-Valued Semi-Separable Integral Kernels Revisited

We revisit the computation of (2-modified) Fredholm determinants for operators with matrix-valued semi-separable integral kernels. The latter occur, for instance, in the form of Greens functions associated with closed ordinary differential operators on arbitrary intervals on the real line. Our approach determines the (2-modified) Fredholm determinants in terms of solutions of closely associated Volterra integral equations, and as a result offers a natural way to compute such determinants.We illustrate our approach by identifying classical objects such as the Jost function for half-line Schrödinger operators and the inverse transmission coe.cient for Schrödinger operators on the real line as Fredholm determinants, and rederiving the well-known expressions for them in due course. We also apply our formalism to Floquet theory of Schrödinger operators, and upon identifying the connection between the Floquet discriminant and underlying Fredholm determinants, we derive new representations of the Floquet discriminant.Finally, we rederive the explicit formula for the 2-modified Fredholm determinant corresponding to a convolution integral operator, whose kernel is associated with a symbol given by a rational function, in a straghtforward manner. This determinant formula represents a Wiener-Hopf analog of Days formula for the determinant associated with finite Toeplitz matrices generated by the Laurent expansion of a rational function.     

A set of Wiener-Hopf integral equations with common solution in fluctuation theory

Given the kernel of a Wiener-Hopf integral equation the solution of which is the distribution of the maximal term among consecutive partial sums of i.i.d. variables, we construct a sequence of integral equations having this solution in common. Subsequently, we examine the question of approximating this solution to within a specified degree of accuracy by means of discrete distributions.