Romanyuk, A. S., and V. S. Romanyuk. “Best Bilinear Approximations of Functions from Nikolskii-Besov Classes”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 64, no. 5, May 2012, pp. 685-97, https://umj.imath.kiev.ua/index.php/umj/article/view/2608.