ROMANYUK, A. S.; ROMANYUK, V. S. Trigonometric and orthoprojection widths of classes of periodic functions of many variables. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 61, n. 10, p. 1348–1366, 2009. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3106. Acesso em: 14 apr. 2025.