Oridoroga, L. L., M. M. Malamud, and A. V. Agibalova. 2012. “On the Completeness of General Boundary Value Problems for $2 \times 2$ First-Order Systems of Ordinary Differential Equations”. Methods of Functional Analysis and Topology 18 (1): 4-18. https://zen.imath.kiev.ua/index.php/mfat/article/view/502.