1. Nonoscillation theory of elliptic equations of order 2n
2. On the equivalence of two types of oscillation for elliptic operators
3. and : Oscillation criteria for elliptic systems, Proceedings of the American Mathematical Society 27, 325–330 (1971).
4. : Advanced Calculus, Second Edition, McGraw-Hill Book Company, New York 1965.
5. and : Positive functionals and oscillation criteria for differential systems, Optimal Control and Differential Equations. Edited by et. al. Academic Press, New York 1978.