Romanyuk, A. S., and V. S. Romanyuk. “Best Bilinear Approximations for the Classes of Functions of Many Variables”. Ukrains’kyi Matematychnyi Zhurnal 65, no. 12 (December 25, 2013): 1681–1699. Accessed April 25, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/2546.