Il’kivV. S., and StrapN. I. “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, Vol. 67, no. 5, May 2015, pp. 611-24, https://umj.imath.kiev.ua/index.php/umj/article/view/2009.