Sidon-type inequalities and strong approximation by Fourier sums in multiplicative systems

Considering the character systems of Vilenkin groups, we obtain some Sidon-type inequalities that generalize the results by Móricz and Avdispahić–Pepić. We apply the inequalities to the theory of strong approximation of functions by series in character systems. Our results are analogs of the trigonometric results by Totik, Leindler, Németh, and Siddiqi.