Shabozov, M. Sh. “Best Approximation and Best Unilateral Approximation of the Kernel of a Biharmonic Equation and Optimal Renewal of the Values of Operators”. Ukrains’kyi Matematychnyi Zhurnal 47, no. 11 (November 25, 1995): 1549–1557. Accessed May 18, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5546.