1. Braunbek, V. W. and G. Laukien, "Einzelheiten zur Halbebenen- Beugung," Optik, 9, 174–179, 1952.
2. Krasnoselskiy, M. A., A. I. Perov, A. I. Povolotskiy, and P. P. Zabreiko, Plane Vector Fields, Academic Press, New York, 1966.
3. Forster, H., "Uber das Verhalten der Integralkurven einer gewohnlichen Differentialgleichung erster Ordung in der Umgebung eines singul¨aren Punktes," Mathematische Zietschrift, 43, 271-320, 1937.
4. Nemytskii, V. V. and V. V. Stepanov, Qualitative Theory of Differential Equations, Princeton University Press, 1960.
5. Hartman, P., Ordinary Differential Equations, John Wiley & Sons, New York, 1964.
6. Latmiral, G., "Curl of Poynting vector and differential radiation pressure," Alta Frequenza, 13, 287-289, 1984.
7. Braunbek, V. W., "Zur Darstellung von Wellenfeldern," Zeitschrift Naturforschung, 6a, 12-15, 1951.