1. Hadamard, J., C. R. Acad. Sci., Paris 152 (1911) 1735; see also H. Lamb, Hydrodynamics, 6th ed., p. 600; Cambridge, 1932.
2. Serret-Scheffers, Lehrbuch der Differential- und Integralrechnung, 4th and 5th ed., vol. 2, p. 78.
3. Carslaw, H. S., and J. C. Jaeger, Conduction of heat in solids, p. 200; Oxford, 1948.
4. Kamke, E., Differentialgleichungen, vol. 1, p. 220; Leipzig, 1942.