Radzievskii, G. V. 2003. “Uniqueness of Solutions of Some Nonlocal Boundary-Value Problems for Operator-Differential Equations on a Finite Segment”.
Ukrains’kyi Matematychnyi Zhurnal 55 (7): 1006-9.
https://umj.imath.kiev.ua/index.php/umj/article/view/3976.