RomanyukA. S., and RomanyukV. S. “Asymptotic Estimates for the Best Trigonometric and Bilinear Approximations of Classes of Functions of Several Variables”. Ukrains’kyi Matematychnyi Zhurnal 62, no. 4 (April 25, 2010): 536–551. Accessed July 22, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/2886.