Velichko, E.V., and I.G. Tkachenko. “On the Correctness of the Determination of Trigonometric Functions by the System of Functional Equations”. Carpathian Mathematical Publications 3, no. 1 (June 30, 2011): 15–21. Accessed May 6, 2024. https://journals.pnu.edu.ua/index.php/cmp/article/view/3081.