Kononovich T. Estimate of best approximations of periodic functions of several variables by Fourier coefficients

To obtain estimations of best approximations by trigonometric polynomials of periodic functions of several variables in terms of Fourier coefficients.The upper bound for the best approximation of functions, representable by double trigonometric series that satisfy conditions for integrability, upper bounds for the following values: the norm, the best approximation by the "angle" and by trigonometric polynomials of functions of the space Lp that are sums of double trigonometric series with certain conditions for coefficients, lower bound for a sum of norms and for a sum of the best approximations of functions of the space L and conjugated by each and both variables functions were obtained