1. Sur une inégalité;Opial;Ann. Polon. Math.,1960
2. Opial Inequalities with Applications in Differential and Difference Equations;Agarwal,1995
3. A simple proof of a certain result of Z. Opial;Olech;Ann. Polon. Math.,1960
4. Analytic Inequalities;Mitrinović,1970
5. Sharp Opial-type inequalities involving higher order derivatives of two functions;Agarwal;Math. Nachr.,1995