convergence of orthogonal series, Convergence and divergence of series and sequences of functions, monotone sequences of real numbers, Fourier series in special orthogonal functions (Legendre polynomials, Walsh functions, etc.), increasing sequences of natural numbers, absolute convergence, Convergence and divergence of series and sequences, Inequalities for sums, series and integrals, Haar series, Orthogonal functions and polynomials, general theory of nontrigonometric harmonic analysis, summation of general orthogonal series, equivalence of coefficient conditions