1. A. Zygmund, Trigonometric Series, Vols. 1 and 2, Cambridge Univ. Press (1968).
2. P. L. Ul'yanov, ?Solved and unsolved problems in the theory of trigonometric and orthogonal series,? Usp. Mat. Nauk,19, 3?69 (1964).
3. F. T. Wang, ?Note on the absolute summability of trigonometrical series,? J. London Math. Soc.,16, 174?176 (1941).
4. T. Tsuchikura, ?Absolute Cesaro summability of orthogonal series,? Tôhoku Math. J.,5, 52?66 (1953).
5. S. B. Stechkin, ?Absolute convergence of orthogonal series,? Usp. Mat. Nauk,2, 177?178 (1947).