Il’kiv, V. S., and N. I. Strap. “Solvability of the Nonlocal Boundary-Value Problem for a System of Differential-Operator Equations in the Sobolev Scale of Spaces and in a Refined Scale”. Ukrains’kyi Matematychnyi Zhurnal 67, no. 5 (May 25, 2015): 611-624. Accessed October 1, 2022. http://umj.imath.kiev.ua/index.php/umj/article/view/2009.