1. Smirnov, V. (1970) Cours
de Mathematiques Superieurs, Tome II, Mir, Moscow
2. Coddington, E. A. and Levinson, N. (1955) Theory of Ordinary Differential Equations, McGraw-Hill, New York
3. Collard, K. and Hall, G. G. (1977) Int. J. Quantum. Chem.
12, 623
4. Bader, R. F. W and Nguyen-Dang, T. T. (1981) Adv. Quantum. Chem.
14, 63, and references therein
5. 2. Lazzeretti, P. Rossi, E. and Zanasi, R. (1984) Int. J. Quantum Chem.