1. G. Alexits,Convergence problems of orthogonal series, Pergamon (Oxford, 1961).
2. J. M. Ash andG. V. Welland, Convergence, uniqueness, and summability of multiple trigonometric series,Trans. Amer. Math. Soc.,163 (1972), 401–436.
3. G. H. Hardy, On the convergence of certain multiple series,Proc. Cambridge Philosoph. Soc.,19 (1916–1919), 86–95.
4. G. E. Reves andO. Szász, Some theorems on double trigonometric series,Duke Math. J.,9 (1942), 693–705.
5. A. Rényi,Probability theory, Akadémiai Kiadó (Budapest, 1970).