1. On an integral inequality of Z. Opial;Beesack;Trans. Amer. Math. Soc.,1962
2. An inequality similar to Opial's inequality;Das,1969
3. An even simpler proof of Opial's inequality;Mallows,1965
4. Analytic Inequalities;Mitrinović,1970
5. Boundary value problems for nonlinear differential and difference equations of the second order;Myjak;Zeszyty Nauk. Uniw. Jagiellon. Prace Mat.,1971