HEMBARS'KA, S. B. Estimates for the Variation of Functions Defined by Double Trigonometric Cosine Series. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 55, n. 6, p. 733–749, 2003. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3949. Acesso em: 5 dec. 2025.