PLESHAKOV, M. G.; POPOV, P. A. Sign-Preserving Approximation of Periodic Functions. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 55, n. 8, p. 1087–1098, 2003. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3983. Acesso em: 5 dec. 2025.