Zhyhallo, T. V., and Yu. I. Kharkevych. “Approximating Properties of Biharmonic Poisson Operators in the Classes $\hat{L}^{\psi}_{\beta, 1}$ ”. Ukrains’kyi Matematychnyi Zhurnal 69, no. 5 (May 25, 2017): 650-656. Accessed April 18, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/1724.