1. Courant, R. and D. Hilbert, Methods of mathematical physics II, Interscience, New York 1962.
2. Broer, L. J. F. and L. A. Peletier, Appl. Sci. Res. 17 (1967) 65.
3. ??Ibid. 17 (1967) 133.
4. Whittaker, E. T. and G. N. Watson, A course of modern analysis, Cambridge University Press, Cambridge 1935.
5. Forsyth, A. R., Theory of functions of a complex variable, 3rd ed., Cambridge University Press, Cambridge 1918.