DeinekaV. S., SergienkoI. V., and SkopetskiiV. V. 1999. “Eigenvalue Problems With Discontinuous Eigenfunctions and Their Numerical Solutions”. Ukrains’kyi Matematychnyi Zhurnal 51 (10), 1317–1323. https://umj.imath.kiev.ua/index.php/umj/article/view/4730.