VELICHKO, I. G.; STEGANTSEVA, P. G. Example of a function of two variables that cannot be an $R$-function. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 62, n. 2, p. 270–274, 2010. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/2863. Acesso em: 5 dec. 2025.