DZYUBENKO, H. A. Comonotone approximation of twice differentiable periodic functions. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 61, n. 4, p. 435–451, 2009. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3032. Acesso em: 16 feb. 2026.