On the coefficients of Fourier series with respect to the Haar system

If a continuous function is expanded according to a complete orthonormal system of discontinuous functions (such as the Walsh and Haar systems), the expansion coefficients are subject not only to upper bounds but to lower bounds as well. In this paper lower bounds are derived for the Haar–Fourier coefficients, valid for different classes of continuous functions, and a connection is established between this kind of inequalities and absolute convergence everywhere of the Haar–Fourier series.
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