1. Comparison theorems for elliptic differential equations;Clark,1965
2. Direct Methods of Qualitative Spectral Analysis of Singular Differential Operators;Glazman,1965
3. On a comparison theorem for self-adjoint partial differential equations of elliptic type;Hartman,1955
4. Oscillation criteria for elliptic equations;Headley;Pacific J. Math.,1968
5. A strong comparison theorem for self-adjoint elliptic equations;Kreith,1968