1. De Vogelaere, R.: 1949,Proc. Sec. Math. Congress Vancouver, pp. 170?171.
2. De Vogelaere, R.: 1950,Can. J. Math. 2, 440.
3. De Vogelaere, R.: 1958,Contributions to the Theory of Non-Linear Oscillations, Vol. IV, pp. 53?84.
4. Dragt, A.: 1965,Rev. Geophys. 3, 255.
5. Graef, C.: 1944,Bol. Soc. Matem. Mexicana 1, 1?31.