Deviations of Fejer Sums and Rates of Convergence in the von Neumann Ergodic Theorem

It turns out that the deviations of the Fejer sums for continuous 2π-periodic functions and the rates of convergence in the von Neumann ergodic theorem can both be calculated using, in fact, the same formulas (by integrating the Fejer kernels). As a result, for many dynamical systems popular in applications, the rates of convergence in the von Neumann ergodic theorem can be estimated with a sharp leading coefficient of the asymptotic by applying S.N. Bernstein’s more than hundred-year old results in harmonic analysis.