Aliev, I. V. “Summability of Series in the Root Functions of Boundary-Value Problems of Bitsadze-Samarskii Type”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 60, no. 4, Apr. 2008, pp. 443–452, https://umj.imath.kiev.ua/index.php/umj/article/view/3167.