A. S. Romanyuk, “Approximation of classes of functions of many variables by their orthogonal projections onto subspaces of trigonometric polynomials”, Ukr. Mat. Zhurn., vol. 48, no. 1, pp. 80–89, Jan. 1996, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5365