VelichkoI. G., and StegantsevaP. G. “Example of a Function of Two Variables That Cannot Be an $R$-Function”. Ukrains’kyi Matematychnyi Zhurnal 62, no. 2 (February 25, 2010): 270–274. Accessed September 1, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/2863.