L. Tepoyan and H. Grigoryan, “ON DEGENERATE NONSELF-ADJOINT DIFFERENTIAL EQUATIONS OF FOURTH ORDER”, Proc. YSU A: Phys. Math. Sci., vol. 46, no. 3 (229), pp. 29–33, Dec. 2012, doi: 10.46991/PYSUA.2012.46.3.029.