LevchukV. M. “On Conditionless Bases of the Kernels Generated by Differential Equations of the second Order”. Ukrains’kyi Matematychnyi Zhurnal 69, no. 3 (March 25, 2017): 332-347. Accessed November 27, 2020. http://umj.imath.kiev.ua/index.php/umj/article/view/1700.