Analysis of the results of a computational experiment to restore the discontinuous functions of two variables using projections

This article presents the main statements of the method of approximation of discontinuous functions of two variables, describing an image of the surface of a 2D body or an image of the internal structure of a 3D body in a certain plane, using projections that come from a computer tomograph. The method is based on the use of discontinuous splines of two variables and finite Fourier sums, in which the Fourier coefficients are found using projection data. The method is based on the following idea: an approximated discontinuous function is replaced by the sum of two functions – a discontinuous spline and a continuous or differentiable function. A method is proposed for constructing a spline function, which has on the indicated lines the same discontinuities of the first kind as the approximated discontinuous function, and a method for finding the Fourier coefficients of the indicated continuous or differentiable function. That is, the difference between the function being approximated and the specified discontinuous spline is a function that can be approximated by finite Fourier sums without the Gibbs phenomenon. In the numerical experiment, it was assumed that the approximated function has discontinuities of the first kind on a given system of circles and ellipses nested into each other. The analysis of the calculation results showed their correspondence to the theoretical statements of the work. The proposed method makes it possible to obtain a given approximation accuracy with a smaller number of projections, that is, with less irradiation.

Exact reconstruction of sparse non-harmonic signals from their Fourier coefficients

In this paper, we derive a new reconstruction method for real non-harmonic Fourier sums, i.e., real signals which can be represented as sparse exponential sums of the form $$f(t) = \sum _{j=1}^{K} \gamma _{j} \, \cos (2\pi a_{j} t + b_{j})$$ f ( t ) = ∑ j = 1 K γ j cos ( 2 π a j t + b j ) , where the frequency parameters $$a_{j} \in {\mathbb {R}}$$ a j ∈ R (or $$a_{j} \in {\mathrm i} {\mathbb {R}}$$ a j ∈ i R ) are pairwise different. Our method is based on the recently proposed numerically stable iterative rational approximation algorithm in Nakatsukasa et al. (SIAM J Sci Comput 40(3):A1494–A1522, 2018). For signal reconstruction we use a set of classical Fourier coefficients of f with regard to a fixed interval (0, P) with $$P>0$$ P > 0 . Even though all terms of f may be non-P-periodic, our reconstruction method requires at most $$2K+2$$ 2 K + 2 Fourier coefficients $$c_{n}(f)$$ c n ( f ) to recover all parameters of f. We show that in the case of exact data, the proposed iterative algorithm terminates after at most $$K+1$$ K + 1 steps. The algorithm can also detect the number K of terms of f, if K is a priori unknown and $$L \ge 2K+2$$ L ≥ 2 K + 2 Fourier coefficients are available. Therefore our method provides a new alternative to the known numerical approaches for the recovery of exponential sums that are based on Prony’s method.

Approximative characteristics of the Nikol'skii-Besov-type classes of periodic functions in the space $B_{\infty,1}$

We obtained the exact order estimates of the orthowidths and similar to them approximative characteristics of the Nikol'skii-Besov-type classes $B^{\Omega}_{p,\theta}$ of periodic functions of one and several variables in the space $B_{\infty,1}$. We observe, that in the multivariate case $(d\geq2)$ the orders of orthowidths of the considered functional classes are realized by their approximations by step hyperbolic Fourier sums that contain the necessary number of harmonics. In the univariate case, an optimal in the sense of order estimates for orthowidths of the corresponding functional classes there are the ordinary partial sums of their Fourier series. Besides, we note that in the univariate case the estimates of the considered approximative characteristics do not depend on the parameter $\theta$. In addition, it is established that the norms of linear operators that realize the order of the best approximation of the classes $B^{\Omega}_{p,\theta}$ in the space $B_{\infty,1}$ in the multivariate case are unbounded.

Approximation Properties of Discrete Fourier Sums in Polynomials Orthogonal on Non-Uniform Grids

В данной работе для произвольной непрерывной на отрезке $[-1, 1]$ функции $f(x)$ в~случае целых положительных $\alpha$ и $\beta$ построены дискретные суммы Фурье $S_{n,N}^{\alpha,\beta}(f,x)$ по системе многочленов $\{\hat{p}_{k,N}^{\alpha,\beta}(x)\}_{k=0}^{N-1},$ образующих ортонормированную систему на неравномерных сетках $\Omega_N=\{x_j\}_{j=0}^{N-1},$ состоящих из конечного числа $N$ точек отрезка $[-1, 1]$ с весом типа Якоби. Исследуются аппроксимативные свойства построенных частных сумм $S_{n,N}^{\alpha,\beta}(f,x)$ порядка $n\leq{N-1}$ в~пространстве непрерывных функциий $C[-1, 1].$ А именно, получена двусторонняя поточечная оценка для функции Лебега $L_{n,N}^{\alpha,\beta}(x)$ рассматриваемых дискретных сумм Фурье при $n=O\big(\delta_N^{-1/(\lambda+3)}\big)$, $\lambda=\max\{\alpha, \beta\}$, $\delta_N=\max_{0\leq{j}\leq{N-1}}\Delta{t_j}$. Соответственно, исследован также вопрос сходимости $S_{n,N}^{\alpha,\beta}(f,x)$ к $f(x)$. В частности, получена оценка отклонения частичной суммы $S_{n,N}^{\alpha,\beta}(f,x)$ от $f(x)$ при $n=O\big(\delta_N^{-1/(\lambda+3)}\big),$ которая также зависит от~$n$ и положения точки $x\in[-1, 1].$

APPROXIMATION OF CLASSES OF POISSON INTEGRALS BY REPEATED FEJER SUMS

The paper is devoted to the approximation by arithmetic means of Fourier sums of classes of periodic functions of high smoothness. The simplest example of a linear approximation of continuous periodic functions of a real variable is the approximation by partial sums of the Fourier series. The sequences of partial Fourier sums are not uniformly convergent over the class of continuous periodic functions. A significant number of works is devoted to the study of other approximation methods, which are generated by transformations of Fourier sums and allow us to construct trigonometrical polynomials that would be uniformly convergent for each continuous function. Over the past decades, Fejer sums and de la Vallee Poussin sums have been widely studied. One of the most important direction in this field is the study of the asymptotic behavior of upper bounds of deviations of linear means of Fourier sums on different classes of periodic functions. Methods of investigation of integral representations of deviations of trigonometric polynomials generated by linear methods of summation of Fourier series, were originated and developed in the works of S.M. Nikolsky, S.B. Stechkin, N.P. Korneichuk, V.K. Dzadyk and others. The aim of the work systematizes known results related to the approximation of classes of Poisson integrals by arithmetic means of Fourier sums, and presents new facts obtained for particular cases. In the paper is studied the approximative properties of repeated Fejer sums on the classes of periodic analytic functions of real variable. Under certain conditions, we obtained asymptotic formulas for upper bounds of deviations of repeated Fejer sums on classes of Poisson integrals. The obtained formulas provide a solution of the corresponding Kolmogorov-Nikolsky problem without any additional conditions.

Asymptotically best possible Lebesgue-type inequalities for the fourier sums on sets of generalized poisson integrals

In this paper we establish Lebesgue-type inequalities for 2?-periodic functions f, which are defined by generalized Poisson integrals of the functions ? from Lp, 1 ? p < 1. In these inequalities uniform norms of deviations of Fourier sums ||f-Sn-1||C are expressed via best approximations En(?)Lp of functions ? by trigonometric polynomials in the metric of space Lp. We show that obtained estimates are asymptotically best possible.