Nedokis, V. A., and Yu. V. Teplinsky. “Limit Theorems in the Theory of Multipoint Boundary-Value Problems”. Ukrains’kyi Matematychnyi Zhurnal, vol. 51, no. 4, Apr. 1999, pp. 519–531, https://umj.imath.kiev.ua/index.php/umj/article/view/4636.