1. Methods of Mathematical Physics I;Courant,1953
2. Comparison theorems for elliptic differential equations;Clark,1965
3. Comparison theorems for linear differential equations of second order;Leighton,1962
4. On a comparison theorem for self-adjoint partial differential equations of elliptic type;Hartman,1955
5. Comparison theorems for constrained rods;Kreith;SIAM Rev.,1964