KomarenkоA. N. “Problem of Eigenvalues With a Parameter under Boundary Value Conditions for Elliptical Equations Degenerating on Part of the Region Boundary ”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 18, no. 6, Oct. 1966, pp. 3-11, https://umj.imath.kiev.ua/index.php/umj/article/view/8455.