Interpolation Formulas for Functions of Exponential Type

In the paper we present a derivative-free estimate of the remainder of an arbitrary interpolation rule on the class of entire functions which, moreover, belong to the space L² _(-,+). The theory is based on the use of the Paley-Wiener theorem. The essential advantage of this method is the fact that the estimate of the remainder is formed by a product of two terms. The first term depends on the rule only while the second depends on the interpolated function only. The obtained estimate of the remainder of Lagrange's rule shows the efficiency of the method of estimate. The first term of the estimate is a starting point for the construction of the optimal rule; only the optimal rule with prescribed nodes of the interpolatory rule is investigated. An example illustrates the developed theory.     

A note on Davis type error bounds

Bounds for the error of the approximation of the value of a bounded linear functionalUf by the weighted sumU _n f= _k ^=0/n–1 a _k f (x _k ) in Hilbert function spacesH(B) based upon the Davis method are considered. Taking into account that functionsf H (B) withUf=U _n f are known, improved error bounds are computed. Applying the explicit formulas numerical examples are treated and comparisons are made.     

The General Problem of Polynomial Spline Interpolation

We study the general problem of interpolation by polynomial splines and consider the construction of such splines using the coefficients of expansion of a certain derivative in B-splines. We analyze the properties of the obtained systems of equations and estimate the interpolation error.     

An Application of Multiple Comparison Techniques to Model Selection

Akaike's information criterion (AIC) is widely used to estimate the best model from a given candidate set of parameterized probabilistic models. In this paper, considering the sampling error of AIC, a set of good models is constructed rather than choosing a single model. This set is called a confidence set of models, which includes the minimum {AIC} model at an error rate smaller than the specified significance level. The result is given as P-value for each model, from which the confidence set is immediately obtained. A variant of Gupta's subset selection procedure is devised, in which a standardized difference of AIC is calculated for every pair of models. The critical constants are computed by the Monte-Carlo method, where the asymptotic normal approximation of AIC is used. The proposed method neither requires the full model nor assumes a hierarchical structure of models, and it has higher power than similar existing methods.     

Superconvergence of mixed finite element methods for optimal control problems

In this paper, we investigate the superconvergence property of the numerical solution of a quadratic convex optimal control problem by using rectangular mixed finite element methods. The state and co-state variables are approximated by the lowest order Raviart-Thomas mixed finite element spaces and the control variable is approximated by piecewise constant functions. Some realistic regularity assumptions are presented and applied to error estimation by using an operator interpolation technique. We derive superconvergence properties for the flux functions along the Gauss lines and for the scalar functions at the Gauss points via mixed projections. Moreover, global superconvergence results are obtained by virtue of an interpolation postprocessing technique. Thus, based on these superconvergence estimates, some asymptotic exactness a posteriori error estimators are presented for the mixed finite element methods. Finally, some numerical examples are given to demonstrate the practical side of the theoretical results about superconvergence.

     

The concept of hyperellipticity on surfaces with nodes

A surface with nodes X is hyperelliptic if there exists an involution such that the genus of X/〈h〉 is 0. We prove that this definition is equivalent, as in the category of surfaces without nodes, to the existence of a degree 2 morphism satisfying an additional condition where the genus of Y is 0. Other question is if the hyperelliptic involution is unique or not. We shall prove that the hyperelliptic involution is unique in the case of stable Riemann surfaces but is not unique in the case of Klein surfaces with nodes. Finally, we shall prove that a complex double of a hyperelliptic Klein surface with nodes could not be hyperelliptic.     

Cubic spline interpolation of functions with high gradients in boundary layers

The cubic spline interpolation of grid functions with high-gradient regions is considered. Uniform meshes are proved to be inefficient for this purpose. In the case of widely applied piecewise uniform Shishkin meshes, asymptotically sharp two-sided error estimates are obtained in the class of functions with an exponential boundary layer. It is proved that the error estimates of traditional spline interpolation are not uniform with respect to a small parameter, and the error can increase indefinitely as the small parameter tends to zero, while the number of nodes N is fixed. A modified cubic interpolation spline is proposed, for which O((ln N/N)⁴) error estimates that are uniform with respect to the small parameter are obtained.     

Error estimates for scattered data interpolation on spheres

We study Sobolev type estimates for the approximation order resulting from using strictly positive definite kernels to do interpolation on the -sphere. The interpolation knots are scattered. Our approach partly follows the general theory of Golomb and Weinberger and related estimates. These error estimates are then based on series expansions of smooth functions in terms of spherical harmonics. The Markov inequality for spherical harmonics is essential to our analysis and is used in order to find lower bounds for certain sampling operators on spaces of spherical harmonics.

     

On the Summability of Trigonometric Interpolation Processes

The aim of this paper is to show that several processes studied in trigonometric interpolation theory can be obtained by -sums of discrete Fourier series. We shall investigate the uniform convergence of the sequences of thesepolynomials. We show that the convergence of several processes can be seen immediately from suitable explicit forms of the corresponding polynomials. Error estimates for the approximation can be also obtained by certain general results.     

The general recurrence relation for divided differences and the general Newton-interpolation-algorithm with applications to trigonometric interpolation

Summary In this note an ultimate generalization of Newton's classical interpolation formula is given. More precisely, we will establish the most general linear form of a Newton-like interpolation formula and a general recurrence relation for divided differences which are applicable whenever a function is to be interpolated by means of linear combinations of functions forming a ebyev-system such that at least one of its subsystems is again a ebyev-system. The theory is applied to trigonometric interpolation yielding a new algorithm which computes the interpolating trigonometric polynomial of smallest degree for any distribution of the knots by recurrence. A numerical example is given.     

Several notes on the circumradius condition

Recently, the so-called circumradius condition (or estimate) was derived, which is a new estimate of the W^1,p-error of linear Lagrange interpolation on triangles in terms of their circumradius. The published proofs of the estimate are rather technical and do not allow clear, simple insight into the results. In this paper, we give a simple direct proof of the p = ∞ case. This allows us to make several observations such as on the optimality of the circumradius estimate. Furthermore, we show how the case of general p is in fact nothing more than a simple scaling of the standard O(h) estimate under the maximum angle condition, even for higher order interpolation. This allows a direct interpretation of the circumradius estimate and condition in the context of the well established theory of the maximum angle condition.     

各向异性网格下二阶椭圆问题的一个混合元形式

提出了二阶椭圆问题的一个混合变分形式,同时证明了Rariart-Thomas元的各向异性插值性质,并给出了单元的对二阶问题的最优误差估计。     

Approximation on the sphere using radial basis functions plus polynomials

In this paper we analyse a hybrid approximation of functions on the sphere by radial basis functions combined with polynomials, with the radial basis functions assumed to be generated by a (strictly) positive definite kernel. The approximation is determined by interpolation at scattered data points, supplemented by side conditions on the coefficients to ensure a square linear system. The analysis is first carried out in the native space associated with the kernel (with no explicit polynomial component, and no side conditions). A more refined error estimate is obtained for functions in a still smaller space. Numerical calculations support the utility of this hybrid approximation.      

Harmonizable stable processes on groups: spectral,ergodic and interpolation properties

Summary This work extends to symmetric -stable (SS) processes, 1<<2, which are Fourier transforms of independently scattered random measures on locally compact Abelian groups, some of the basic results known for processes with finite second moments and for Gaussian processes. Analytic conditions for subordination of left (right) stationarily related processes and a weak law of large numbers are obtained. The main results deal with the interpolation problem. Characterization of minimal and interpolable processes on discrete groups are derived. Also formulas for the interpolator and the corresponding interpolation error are given. This yields a solution of the interpolation problem for the considered class of stable processes in this general setting.Research supported by AFOSR contract F49620 82 C0009On leave from the Institute of Mathematics, Technical University 50-370 Wrocaw, Poland     

Estimates for functions in Sobolev spaces defined on unbounded domains

Given a function u belonging to a suitable Beppo–Levi or Sobolev space and an unbounded domain Ω in , we prove several Sobolev-type bounds involving the values of u on an infinite discrete subset A of Ω. These results improve the previous ones obtained by Madych and Potter [W.R. Madych, E.H. Potter, An estimate for multivariate interpolation, J. Approx. Theory 43 (1985) 132–139] and Madych [W.R. Madych, An estimate for multivariate interpolation II, J. Approx. Theory 142 (2006) 116–128].     

Birkhoff interpolation on non-uniformly distributed roots of unity

This paper studies some cases of (0,m)-interpolation on non-uniformly distributed roots of unity that were not covered before. The interpolation problem uses as nodes the zeros of (z ^k +1)(z ³–1) with k=3n+1, 3n+2. Proof of the regularity is more intricate than when k is divisible by 3, the case included in a previous paper by the authors. The interpolation problem appears to be regular for mk+3, a result that is in tune with the case k=3n mentioned before. However, it is necessary to treat the full general 18×18 linear system. For small values of m the determinant is calculated explicitly using MAPLE V, Release 5.     

Asymptotic theory of parameter estimation by a contrast function based on interpolation error

Interpolation is an important issue for a variety fields of statistics (e.g., missing data analysis). In time series analysis, the best interpolator for missing points problem has been investigated in several ways. In this paper, the asymptotics of a contrast function estimator defined by pseudo interpolation error for stationary process are investigated. We estimate parameters of the process by minimizing the pseudo interpolation error written in terms of a fitted parametric spectral density and the periodogram based on observed stretch. The estimator has the consistency and asymptotical normality. Although the criterion for the interpolation problem is known as the best in the sense of smallest mean square error for past and future extrapolation, it is shown that the estimator is asymptotically inefficient in general parameter estimation, which leads to an unexpected result.     

Error estimates on anisotropic elements for functions in weighted Sobolev spaces

In this paper we prove error estimates for a piecewise average interpolation on anisotropic rectangular elements, i.e., rectangles with sides of different orders, in two and three dimensions.

Our error estimates are valid under the condition that neighboring elements have comparable size. This is a very mild assumption that includes more general meshes than those allowed in previous papers. In particular, strong anisotropic meshes arising naturally in the approximation of problems with boundary layers fall under our hypotheses.

Moreover, we generalize the error estimates allowing on the right-hand side some weighted Sobolev norms. This extension is of interest in singularly perturbed problems.

Finally, we consider the approximation of functions vanishing on the boundary by finite element functions with the same property, a point that was not considered in previous papers on average interpolations for anisotropic elements.

As an application we consider the approximation of a singularly perturbed reaction-diffusion equation and show that, as a consequence of our results, almost optimal order error estimates in the energy norm, valid uniformly in the perturbation parameter, can be obtained.

     

On Some Interpolation Rules for Lattice Ordered Groups

Let be an infinite cardinal. In this paper we define an interpolation rule IR() for lattice ordered groups. We denote by C() the class of all lattice ordered groups satisfying IR(), and prove that C() is a radical class.