ROMANYUK, A. S. Approximation of classes of functions of many variables by their orthogonal projections onto subspaces of trigonometric polynomials. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 48, n. 1, p. 80–89, 1996. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5365. Acesso em: 5 dec. 2025.