Presented to the Society, December, 1955 under the title A necessary condition for nonoscillation of a system of second order differential equations, and August, 1956; received by the editors August 2, 1956. 1 These results were obtained while the author held a National Science Foundation grant, NSF-G1825 and was at Yale University. Now at the University of Utah. I This paper contains a bibliography of the use of (2). To this list should be added paper [3] which appeared almost simultaneously with [1].