1. J. H. Barrett, Systems-disconjugacy of a fourth-order differential equation,Proc. Amer. Math. Soc. 12 (1961), 205–213.
2. J. H. Barrett, Two-point boundary value problems and comparison theorems for fourth-order self-adjoint differential equations and second-order matrix differential systems,Canad. J. Math. 13 (1961), 625–638.
3. M. Greguš, Über die lineare homogene Differentialgleichung dritter Ordnung,Wiss. Z. Martin-Luther-Univ. Halle—Wittenberg Math.-Natur. Reihe 12 (1963), 265–286.
4. M. Greguš, Über die Eigenschaften der Lösungen einiger quasilinearer Gleichungen 3. Ordnung,Acta Fac. Rerum Natur. Univ. Comenian. Math. 12 (1965), 11–22.
5. M. Greguš, On linear differential equations of higher odd order,Acta Fac. Rerum Natur. Univ. Comenian. Math. 17 (1967), 81–88.