On approximation of functions by trigonometric polynomials with incomplete spectrum in <Emphasis Type="Italic">L</Emphasis><Subscript><Emphasis Type="Italic">p</Emphasis></Subscript>, 0 < <Emphasis Type="Italic">p</Emphasis> < 1

Let B be a set of integers with certain arithmetic properties. We obtain estimates of the best approximation of functions in the space L _p , 0 < p <1, by trigonometric polynomials that are constructed by the system {e^ikx}_{k ? \mathbbZ\B} \{e^{ikx}\}_{k\in \mathbb{Z}\backslash B} . Bibliography: 13 titles.     

On trigonometric polynomials least deviating from zero

We give a solution of the problem about trigonometric polynomials with a given leading harmonic and least deviating from zero in measure; more precisely, with respect to the functional μ(f _n ) = mes {t ∈ [0, 2π]: |f _n (t)| ≥ 1}. We give a solution of a related problem about the minimal value over compact sets (from the real line) of a given measure of least uniform deviation from zero on a compact set for trigonometric polynomials with a fixed leading harmonic. Published in Russian in Doklady Akademii Nauk, 2009, Vol. 425, No. 6, pp. 733–736. Presented by Academician A.M. Il’in November 13, 2008 The article was translated by the authors.     

Hermite interpolation with trigonometric polynomials

Interpolation methods of Hermite type in translation invariant spaces of trigonometric polynomials for any position of interpolation points and any number of derivatives are constructed. For the case of an odd number of interpolation conditions-periodic trigonometric polynomials of minimum order are chosen as interpolation functions while for the case of an even number of interpolation conditions-antiperiodic trigonometric polynomials of minimum order are appropriate.     

Best time localized trigonometric polynomials and wavelets

In spaces of trigonometric polynomials, the minimum of the angular variance is determined, which is a time localization measure forL ₂ ² . Wavelets and wavelet packets are constructed with the resulting polynomials.     

Some extremal problems for trigonometric polynomials

Three extremal problems for trigonometric polynomials are studied in this paper. The first was initiated by Maiorov. It relates to the trigonometric polynomials with n nonzero harmonics. The L_p-norm of the Weyl derivative is compared with the L_q-norm of the polynomial. The other two problems have appeared in the recent paper by Oswald. They deal with the polynomials of degree n. The minimum of L_p-norm with respect to the choice of phases is compared with l_q-norm of its coefficients.     

On Bernstein–Markov-type inequalities for multivariate polynomials in -norm

Bernstein–Markov-type inequalities provide estimates for the norms of derivatives of algebraic and trigonometric polynomials. They play an important role in Approximation Theory since they are widely used for verifying inverse theorems of approximation. In the past decades these inequalities were extended to the multivariate setting, but the main emphasis so far was on the uniform norm. It is considerably harder to derive Bernstein–Markov-type inequalities in the L_q-norm, and it requires introduction of new methods. In this paper we verify certain Bernstein–Markov-type inequalities in L_q-norm on convex and star-like domains. Special attention is given to the question of how the geometry of the domain affects the corresponding estimates.     

Sharp inequalities for trigonometric polynomials with respect to integral functionals

The problem on sharp inequalities for linear operators on the set of trigonometric polynomials with respect to integral functionals \(\int_0^{2\pi } {\phi \left( {\left| {f\left( x \right)} \right|} \right)dx}\) is discussed. A solution of the problem on trigonometric polynomials with given leading harmonic of least deviation from zero with respect to such functionals over the set of all functions φ defined, nonnegative, and nondecreasing on the semiaxis [0,+∞) is given.     

Some Bernstein-type inequalities for trigonometric polynomials

The refinements of some well-known Bemstein-type inequalities for trigonometric polynomials are obtained.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 45, No. 3, pp. 428–430, March, 1993.     

Nikol'skij-type and maximal inequalities for generalized trigonometric polynomials

We consider some Nikol'skij-type inequalities, thus inequalities between different metrics of a function, for almost periodic trigonometric polynomials. Some basic methods of probability theory are applied to prove the existence of the distribution function for an almost periodic function in the sense of Besicovitch. Finally, the Maximal function of Hardy and Littlewood is considered and maximal inequalities on Besicovitch spaces are proved. Received: 23 July 1998 / Revised version: 8 March 1999     

The <Emphasis Type="Italic">(S)</Emphasis><Stack><Subscript>+</Subscript><Superscript>1</Superscript></Stack>Condition for Generalized Vector Variational Inequalities

In this paper, we extend the (S) ₊ ¹ condition to multivalued mappings in an ordered Hausdorff topological vector space and we derive some existence results for generalized vector variational inequalities associated with multivalued mappings satisfying the (S) ₊ ¹ condition. We generalize also an existence result of Cubiotti and Yao for generalized variational inequalities of class (S) ₊ ¹ to barreled normed spaces. As consequences, some existence results for vector variational inequalities are established.This work was partially supported by grants from the National Science Council of the Republic of China. Communicated by H. P. Benson     

On the best approximation of classes of convolutions of periodic functions by trigonometric polynomials

We present sufficient conditions for kernels to belong to the classN _n ^* . In certain cases, this enables us to find exact values of the best approximations of classes of convolutions by trigonometric polynomials in the metrics ofC andL.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 47, No. 9, pp. 1261–1265, September, 1995.     

Deterministic sampling of sparse trigonometric polynomials

One can recover sparse multivariate trigonometric polynomials from a few randomly taken samples with high probability (as shown by Kunis and Rauhut). We give a deterministic sampling of multivariate trigonometric polynomials inspired by Weil’s exponential sum. Our sampling can produce a deterministic matrix satisfying the statistical restricted isometry property, and also nearly optimal Grassmannian frames. We show that one can exactly reconstruct every M-sparse multivariate trigonometric polynomial with fixed degree and of length D from the determinant sampling X, using the orthogonal matching pursuit, and with |X| a prime number greater than (MlogD)². This result is optimal within the (logD)² factor. The simulations show that the deterministic sampling can offer reconstruction performance similar to the random sampling.     

Nikol’skii-Stechkin inequality for trigonometric polynomials in L 0

In this paper, the Nikol’skii-Stechkin inequality for the trigonometric polynomials is generalized to the space L ₀. The resulting estimates are final.     

Kernels and best approximations related to the system of ultraspherical polynomials

We study the uniformly bounded orthonormal system of functions

where is the normalized system of ultraspherical polynomials. We investigate some approximation properties of the system and we show that these properties are similar to one's of the trigonometric system. First, we obtain estimates of L^p-norms of the kernels of the system . These estimates enable us to prove Nikol'skiı˘-type inequalities for -polynomials. Next, we prove directly that is a basis in each , where w is an arbitrary A_p-weight function. Finally, we apply these results to get sharp inequalities for the best -approximations in L^q in terms of the best -approximations in . For the trigonometric system such inequalities have been already known.     

A weak-type inequality for uniformly bounded trigonometric polynomials

This paper is devoted to refining the Bernstein inequality. Let D be the differentiation operator. The action of the operator Λ = D/n on the set of trigonometric polynomials T _n is studied: the best constant is sought in the inequality between the measures of the sets {x ∈ T: |Λt(x)| > 1} and {x ∈ T: |t(x)| > 1}. We obtain an upper estimate that is order sharp on the set of uniformly bounded trigonometric polynomials T _n ^C = {t ∈ T _n : ‖t‖ ≤ C}.     

Extremal properties of certain trigonometric functions and Chebyshev polynomials

For a wide class of symmetric trigonometric polynomials, the minimal deviation property is established. As a corollary, the extremal property is proved for the components of the Chebyshev polynomial mappings corresponding to symmetric algebras A _α.     

On the completeness of algebraic polynomials in the spaces <Emphasis Type="Italic">L</Emphasis><Subscript><Emphasis Type="Italic">p</Emphasis></Subscript>(ℝ, <Emphasis Type="Italic">d</Emphasis>μ)

We prove that the theorem on the incompleteness of polynomials in the space C ₀ ^w established by de Branges in 1959 is not true for the space L _p (ℝ, dμ) if the support of the measure μ is sufficiently dense.     

Approximation of infinitely differentiable periodic functions by interpolation trigonometric polynomials

We establish asymptotically unimprovable interpolation analogs of Lebesgue-type inequalities on the classes of periodic infinitely differentiable functions C C whose elements can be represented in the form of convolutions with fixed generating kernels. We obtain asymptotic equalities for upper bounds of approximations by interpolation trigonometric polynomials on the classes C _, and C H .Translated from Ukrainskyi Matematychnyi Zhurnal, Vol. 56, No. 4, pp. 495–505, April, 2004.     

On the approximation of entire functions by trigonometric polynomials

Let a set B have the following properties: if z ∈ B, then z ± 2π ∈ B and the intersection of B with the vertical strip 0 ≤ Re x ≤ π is a closed and bounded set. In this paper we study the approximation of a continuous on B and 2π-periodic function f(z) by trigonometric polynomials T _n (z). We establish the necessary and sufficient conditions for the function f(z) to be entire and specify a formula for calculating its order. In addition, we describe some metric properties of periodic sets in a plane.