RomanyukA. S. “Approximation of Classes of Functions of Many Variables by Their Orthogonal Projections onto Subspaces of Trigonometric Polynomials”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 48, no. 1, Jan. 1996, pp. 80-89, https://umj.imath.kiev.ua/index.php/umj/article/view/5365.