Solich, K. V. “Best Bilinear Approximations of the Classes $S^{\Omega}_{p, \theta}B$ of Periodic Functions of Many Variables”. Ukrains’kyi Matematychnyi Zhurnal 63, no. 6 (June 25, 2011): 809–826. Accessed December 5, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/2764.