Summation methods for Fourier series with respect to the Azoff–Shehada system

A special class of complete minimal systems with complete biorthogonal system in a Hilbert space is considered. This class was introduced by Azoff and Shehada. The paper studies conditions under which there exists a linear summation method for Fourier series with respect to the Azoff–Shehada system. A construction of a linear summation method of the Fourier series for a given vector is presented, as well as a construction of a universal linear summation method.