1. B. S. Kashin and A. A. Saakyan, Orthogonal Series (Amer. Math. Soc., Providence, 1989).
2. G. H. Hardy, “On the convergence of certain multiple series”, Proc. Cambridge Philos. Soc., 19, 86–95, 1916–1919.
3. F. Moricz, “On the convergence in a restricted sense of multiple series”, Anal. Math., 5(2), 135–147, 1979.
4. M. de Guzmán, Differentiation of integrals in ℝn, Lecture Notes in Math., vol. 481 (Springer, Berlin-New York, 1975).
5. T. Sh. Zerekidze, “Convergence of multiple Fourier-Haar series and strong differentiablity of integrals”, Trudy Tbilis. Mat. Inst. Razmadze, 76, 80–99, 1985.