1. [1] O. Dosly, J. Komenda, Conjugacy criteria and principal solutions of self-adjoint linear differential equations, Arch. Math. 31 (1995), 217-238.
2. [2] P. Drabek, A. Kufner, F. Nicolosi, Nonlinear Elliptic Equations--Singular and Degenerate Case, West Bohemia Univ. Press, Pilsen, 1996.
3. [3] A. Elbert, Asymptotic behaviour of autonomous half-linear differential systems on the plane, Studia Sci. Math. Hungar. 19 (1984), 447-464.
4. [4] A. Elbert, A half-linear second order differential equation, In Theory of Differential Equation, Colloq. Math. Soc. Janos Bolyai 30 (1979), 158-180.
5. [5] A. Elbert, Oscillation and nonoscillation theorems for some non-linear ordinary differential equations, Lectures Notes in Math. No. 964 (1982), 187-212.