VELICHKO, E.V.; TKACHENKO, I.G. On the correctness of the determination of trigonometric functions by the system of functional equations. Carpathian Mathematical Publications, [S. l.], v. 3, n. 1, p. 15–21, 2011. Disponível em: https://journals.pnu.edu.ua/index.php/cmp/article/view/3081. Acesso em: 6 may. 2024.