ZHYHALLO, K. M.; KHARKEVYCH, Yu. I. On the Approximation of Functions of the Hölder Class by Triharmonic Poisson Integrals. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 53, n. 6, p. 855–859, 2001. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4306. Acesso em: 6 dec. 2025.