Zhyhallo, T. V., and Y. I. Kharkevych. “Approximating Properties of Biharmonic Poisson Operators in the Classes $\hat{L}^{\psi}_{\beta, 1}$ ”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 69, no. 5, May 2017, pp. 650-6, http://umj.imath.kiev.ua/index.php/umj/article/view/1724.