1. M.V. Keldysh, “On some cases of degenerating of elliptic type equations on the boundary of domain”, Dokl. AN SSSR, 23(2), 181–184, 1951.
2. A.A. Dezin, “Degenerate operator equations”, Mat. Sb., 115(157), (3)(7), 323–336, 1981.
3. V.V. Kornienko, “On the spectrumof degenerate operator equations”, Mat. zametki, 68,(5), 677–691, 2000.
4. L.P. Tepoyan, “Degenerate fourth-order differential-operator equations”, Differ.Uravn., 23(8), 1366–1376, 1987.
5. L.P. Tepoyan, “Degenerate differential-operator equations on infinite intervals”, Journal of Mathematical Sciences, 189(1), 164–172, 2013.