Tepoyan, L.P., and H.S. Grigoryan. “ON DEGENERATE NONSELF-ADJOINT DIFFERENTIAL EQUATIONS OF FOURTH ORDER”. Proceedings of the YSU A: Physical and Mathematical Sciences, vol. 46, no. 3 (229), Dec. 2012, pp. 29-33, https://doi.org/10.46991/PYSUA.2012.46.3.029.