1. and , Oscillation criteria for elliptic systems. Proceedings of the American Mathematical Society. Volume 27, 325–330 (1971).
2. The Oscillation of Elliptic Systems
3. and , Positive functionals and oscillation criteria for differential systems. Optimal Control and Differential Equations. Academic Press, New York 1978, 245–275.
4. and , Calculus of Variations. Prentice-Hall, Englewood Cliffs, New Jersey 1963.
5. and , Comparison and oscillation criteria for selfadjoint vectormatrix differential equations. Pac. J. of Math.