1. Direct Methods of Qualitative Spectral Analysis of Singular Differential Operators;Glazman,1965
2. Nonoscillation theorems;Hille;Trans. Amer. Math. Soc.,1948
3. A criterion for n-n oscillations in differential equations or order 2n;Hinton,1968
4. On self-adjoint differential equations of second order;Leighton;J. London Math. Soc.,1952
5. The existence of conjugate points for self-adjoint differential equations of even order;Lewis,1976