1. A. A. Talalyan, On the convergence almost everywhere of subsequences of partial sums of general orthogonal series, Akad. Nauk Armyan SSR. Izv. Fiz.-Mat. Estest. Tehn. Nauki vol. 10 (1957) pp. 17-34 (Russian, Armenian summary).
2. N. Dunford and J. T. Schwartz, Linear operators I, New York, Interscience, 1958, p. 329.