Dymarskii, Ya. M. “On Manifolds of Self-Adjoint Elliptic Operators With Multiple Eigenvalues”. Methods of Functional Analysis and Topology 7, no. 2 (June 25, 2001): 68–74. Accessed December 17, 2025. https://zen.imath.kiev.ua/index.php/mfat/article/view/151.