Kalchuk, I. V., and Yu. I. Kharkevych. 2007. “Asymptotics of the Values of Approximations in the Mean for Classes of Differentiable Functions by Using Biharmonic Poisson Integrals”. Ukrains’kyi Matematychnyi Zhurnal 59 (8): 1105–1115. https://umj.imath.kiev.ua/index.php/umj/article/view/3373.