Oridoroga, L. L., M. M. Malamud, and A. V. Agibalova. “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, no. 1 (March 25, 2012): 4–18. Accessed February 4, 2026. https://zen.imath.kiev.ua/index.php/mfat/article/view/502.