Izvekov, I. G., and E. V. Martynenko. “On Gevrais Classes of Certain Self-Adjoint Differential Operators With Degeneration”. Ukrains’kyi Matematychnyi Zhurnal, vol. 45, no. 12, Dec. 1993, pp. 1622–1626, https://umj.imath.kiev.ua/index.php/umj/article/view/5969.