Abstract:
Sharp estimates are obtained for the convergence rate of double Fourier series in terms of general orthogonal polynomials in some classes of functions and for the Kolmogorov $N$-widths of these classes. These results find applications in numerical analysis.
Key words:orthogonal polynomials, double Fourier series, generalized modulus of continuity, Kolmogorov $N$-width, convergence rate estimate for double Fourier series.