1. Lectures on Elliptic Boundary Value Problems;Agmon,1965
2. A comparison theorem for nonlinear operators;Allegretto;Ann. Scuola Norm. Sup. Pisa,1971
3. Teoremi di confronto fra equazioni o sistemi di equazioni differenziali lineari del second'ordine;Cimmino,1936
4. Comparison theorems for elliptic differential equations;Clark,1965