Zhyhallo, T. V., and Yu. I. Kharkevych. 2017. “Approximating Properties of Biharmonic Poisson Operators in the Classes $\hat{L}^{\psi}_{\beta, 1}$ ”. Ukrains’kyi Matematychnyi Zhurnal 69 (5), 650-56. https://umj.imath.kiev.ua/index.php/umj/article/view/1724.