基于Samelson逆的向量值连分式的收敛准则

The aim of this work is to give some criteria on the convergence of vector valued continued fractions defined by Samelson inverse. We give a new approach to prove the convergence theory of continued fractions. First, by means of the modified classical backward recurrence relation, we obtain a formula between the m-th and n-th convergence of vector valued continued fractions. Second, using this formula, we give necessary and sufficient conditions for the convergence of vector valued continued fractions.     

N-point Padé approximants to real-valued Stieltjes series with nonzero radii of convergence

By employing special continued fractions to two Stieltjes series with nonzero radii of convergence we extend the inequalities for one-point Padé approximants reported by Baker (1975, Corollory 17.1) to the case of two-point Padé approximants. We prove that some convergents of the continued fractions form a monotone sequences of upper and lower bounds converging uniformly to Stieltjes function x₁(x) on compact subsets of (−R, ∞), where R is a radius of convergence of an expansion of x f₁(x) at x = 0. For an illustration of theoretical results we provide nontrivial numerical examples. As an application to real physical problems second order Padé approximants' bounds on the effective conductivity of a square array of cylinders are evaluated.     

Convergence of quantum systems on grids

We discuss here the convergence of quantum systems on grids embedded in Rd and generalize the earlier results found for scalar-valued potentials to the case of matrix-valued potentials. We also discuss the essential self-adjointness of Schrödinger operators for a large class of matrix potentials and give a Feynman-Kac formula for their associated imaginary time Schrödinger semigroups when the matrix potential is positive and continuous. Furthermore, we establish an operator kernel estimate for the semigroups.     

On the Convergence of Bounded J-Fractions on the Resolvent Set of the Corresponding Second Order Difference Operator

We study connections between continued fractions of type J and spectral properties of second order difference operators with complex coefficients. It is known that the convergents of a bounded J-fraction are diagonal Padé approximants of the Weyl function of the corresponding difference operator and that a bounded J-fraction converges uniformly to the Weyl function in some neighborhood of infinity. In this paper we establish convergence in capacity in the unbounded connected component of the resolvent set of the difference operator and specify the rate of convergence. Furthermore, we show that the absence of poles of Padé approximants in some subdomain implies already local uniform convergence. This enables us to verify the Baker–Gammel–Wills conjecture for a subclass of Weyl functions. For establishing these convergence results, we study the ratio and the nth root asymptotic behavior of Padé denominators of bounded J-fractions and give relations with the Green function of the unbounded connected component of the resolvent set. In addition, we show that the number of “spurious” Padé poles in this set may be bounded.     

Ideas from Continued Fraction Theory Extended to Padé Approximation and Generalized Iteration

This is a survey of some basic ideas in the convergence theory for continued fractions, in particular value sets, general convergence and the use of modified approximants to obtain convergence acceleration and analytic continuation. The purpose is to show how these ideas apply to some other areas of mathematics. In particular, we introduce {w _k }-modifications and general convergence for sequences of Padé approximants.     

Approximants of Śleszyński–Pringsheim continued fractions

We characterize the sequences {z_n} of complex numbers which are sequences of approximants of continued fractions K(a_n/b_n) with |a_n|+1⩽|b_n|, and study some of their properties. In particular we give truncation error bounds for such continued fractions.     

Complex Jacobi matrices

Complex Jacobi matrices play an important role in the study of asymptotics and zero distribution of formal orthogonal polynomials (FOPs). The latter are essential tools in several fields of numerical analysis, for instance in the context of iterative methods for solving large systems of linear equations, or in the study of Padé approximation and Jacobi continued fractions. In this paper we present some known and some new results on FOPs in terms of spectral properties of the underlying (infinite) Jacobi matrix, with a special emphasis to unbounded recurrence coefficients. Here we recover several classical results for real Jacobi matrices. The inverse problem of characterizing properties of the Jacobi operator in terms of FOPs and other solutions of a given three-term recurrence is also investigated. This enables us to give results on the approximation of the resolvent by inverses of finite sections, with applications to the convergence of Padé approximants.     

Anti-Gaussian Padé Approximants

This paper gives a synthesis of Padé approximants and anti-Gaussian quadratures. New rational approximants for Stieltjes series have been constructed. In addition, a three term recurrence relation is given for the numerator and denominator, which is useful when the given functional is not defin ite positive.We give the different algebraic properties of these new polynomials, which are similar to those obtained with the Gaussian quadrature formula. We find an easy definition and several relations with Padé approximants. Finally, some numerical results are given in the last section.     

Row convergence theorems for generalised inverse vector-valued Padé approximants

The approximants mentioned in the title are related to vector-valued continued fractions and the vector ϵ-algorithm devised by Wynn in 1963. Here we establish a unitary invariance property of these approximants and describe how the classical (1-dimensional) Padé approximants can be obtained as a special case. The main results of the paper consist of De Montessus—De Ballore type convergence theorems for row sequences (having fixed denominator degree) of vector-valued approximants to meromorphic vector functions.     

ON THE GENERALIZED INVERSE NEVILLE—TYPE MATRIX—VALUED RATIONAL INTERPOLANTS

A new kind of matrix-valued rational interpolants is recursively established by means of generalized Samelson iverse for matrices,with scalar numerator and matrix-valued denominatror.In this respect,it is essentially different form that of the previous works [7,9],where the matrix-valued rational interpolants is in Thiele-type continued fraction form with matrix-valued numerator and scalar denominator.For both univariate and bivariate cases,sufficient conditions for existence,characterisation and univquenese in some sense are proved respectively,and an error formula for the univariate interpolating function is also given.The results obtained in this paper are illustrated with some numerical examples.     

Convergence in measure with respect to a matrix-valued measure and some matrix completion problems

For a matrix-valued measure M we introduce a notion of convergence in measure M, which generalizes the notion of convergence in measure with respect to a scalar measure and takes into account the matrix structure of M. Let S be a subset of the set of matrices of given size. It is easy to see that the set of S-valued measurable functions is closed under convergence in measure with respect to a matrix-valued measure if and only if S is a ρ-closed set, i.e. if and only if SP is closed for any orthoprojector P. We discuss the behaviour of ρ-closed sets under operations of linear algebra and the ρ-closedness of particular classes of matrices.     

A generalized duality method for solving variational inequalities Applications to some nonlinear Dirichlet problems

Summary. In [2] Bermúdez and Moreno introduced a duality algorithm for the numerical solution of variational inequalities; this algorithm is based on some properties of the Yosida regularization of maximal monotone operators. The performances of this algorithm strongly depend on the choice of two constant parameters. A generalization of the algorithm with automatic choice of parameters was discussed in [13], where the constant parameters were replaced by scalar functions, thus improving the convergence of the algorithm. In this article we present a generalization of the Bermúdez-Moreno algorithm that allows the use of very general operators as parameters, extending some of the results in [2], [13] and [14]. As a particular case, we analyze the use of scalar and matrix-valued parameters in a L^p()^M context. We apply the results developed to some boundary value problems involving the p-Laplacian operator, where it is shown that the use of matrix-valued parameters improves the convergence of the algorithm.Mathematics Subject Classification (1991): 47J20 – 65N12     

Some convergence results for the generalized Padé-type approximants

The aim of this paper is to give some convergence results for some sequences of generalized Padé-type approximants. We will consider two types of interpolatory functionals: one corresponding to Langrange and Hermite interpolation and the other corresponding to orthogonal expansions. For these two cases we will give sufficient conditions on the generating functionG(x, t) and on the linear functionalc in order to obtain the convergence of the corresponding sequence of generalized Padé-type approximants. Some examples are given.     

Convergence of matrix continued fractions

The aim of this work is to give some criteria on the convergence of matrix continued fractions. We begin by presenting some new results which generalize the links between the convergent elements of real continued fractions. Secondly, we give necessary and sufficient conditions for the convergence of continued fractions of matrix arguments. This paper will be completed by illustrating the theoretical results with some examples.     

Repeated modifications of limitk-periodic continued fractions

Summary The advantages of using modified approximants for continued fractions, can be enhanced by repeating the modification process. IfK(a _n /b _n) is limitk-periodic, a natural choice for the modifying factors is ak-periodic sequence of right or wrong tails of the correspondingk-periodic continued fraction, if it exists. If the modified approximants thus obtained are ordinary approximants of a new limitk-periodic continued fraction, we repeat the process, if possible. Some examples where this process is applied to obtain a convergence acceleration are also given.     

On two-point Padé-Type and two-Point Padé approximants

Summary Two-point Padé-type approximants are introduced in the case of a non-commutative algebra on a commutative field. Algebraic properties are given and a study of the error of approximation is done. From the relation of the error and some additional properties, two-point Padé approximants are found. Algebraic properties and recurrence relations are proved. The means to compute these approximants in following any way in the table of the approximants are given. The mixed table is introduced, as well as the normality and some results of convergence of two-point Padé-type and Padé approximants.     

On Worpitzky-like theorems for a two-dimensional continued fraction

Using estimates for the remainders of a two-dimensional continued fraction, the relation for the difference between two approximants of such a fraction in terms of these remainders, and the majorant method, we have proposed generalizations of the Worpitzky convergence theorem. For fractions satisfying the conditions of generalized Worpitzky theorems, we have also obtained estimates for their convergence rate.     

Convergence criteria for branched continued fractions with nonnegative components

For branched continued fractions with nonnegative components and a fixed or variable number of branchings we establish necessary and sufficient conditions for their approximants to be well-defined. We study necessary and sufficient conditions for convergence that are multivariable analogs of the known Seidel-Stern and Stern criteria for continued fractions with positive elements. Translated fromMatematichni Metodi ta Fiziko-Mekhanichni Polya, Vol. 40, No. 2, 1997, pp. 7–13.     

Vector greedy algorithms

Our objective is to study nonlinear approximation with regard to redundant systems. Redundancy on the one hand offers much promise for greater efficiency in terms of approximation rate, but on the other hand gives rise to highly nontrivial theoretical and practical problems. Greedy-type approximations proved to be convenient and efficient ways of constructing m-term approximants. We introduce and study vector greedy algorithms that are designed with aim of constructing mth greedy approximants simultaneously for a given finite number of elements. We prove convergence theorems and obtain some estimates for the rate of convergence of vector greedy algorithms when elements come from certain classes.