
In this paper some aspects of approximation of periodic Besov classes \(B_{p,\theta}^r= B_{p,\theta}^r(\pi_d)\), \(\pi_d=\prod_{j=1}^d[-\pi,\pi]\), \(1\leq p\leq \infty\), by linear methods, and of their best approximation by trigonometric polynomials with numbers \(k=(k_1,\dots,k_d)\) of harmonics \(e^{i(k,x)}\) from the stairwise hyperbolic cross in the space \(L_\infty(\pi_d)\) are considered. The behaviour of the norms of the sequence of the linear operators is investigated which guarantee the same order of approximation of the classes \(B_{p,\theta}^r\) in the uniform metric as their best approximations. It is shown that approximation of the classes \(B_{p,\theta}^r\), \(1<\theta<\infty\), \(1\leq p\leq 2\), by trigonometric polynomials via the linear methods in the uniform metric does not realize the order estimates of the corresponding best approximations.
Best approximation, Chebyshev systems, best approximation by trigonometric polynomials, linear approximation methods, Fourier series and coefficients in several variables, de la Vallée Poussin kernel, Approximation by arbitrary linear expressions, Trigonometric approximation, hyperbolic cross, Function spaces arising in harmonic analysis, Sobolev spaces and other spaces of ``smooth'' functions, embedding theorems, trace theorems
Best approximation, Chebyshev systems, best approximation by trigonometric polynomials, linear approximation methods, Fourier series and coefficients in several variables, de la Vallée Poussin kernel, Approximation by arbitrary linear expressions, Trigonometric approximation, hyperbolic cross, Function spaces arising in harmonic analysis, Sobolev spaces and other spaces of ``smooth'' functions, embedding theorems, trace theorems
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