Velichko IG, Stegantseva PG. Example of a function of two variables that cannot be an $R$-function. Ukr. Mat. Zhurn. [Internet]. 2010 Feb. 25 [cited 2025 Dec. 5];62(2):270–274. Available from: https://umj.imath.kiev.ua/index.php/umj/article/view/2863