1. Merlini, D. and Gruber, C., J. math. Phys., to be published.
2. Gruber, C., Merlini, D. and Greenberg, W., preprint.
3. Benettin, G., Jona-Lasinio, G. and Stella, A., preprint.
4. Hintermann, A. and Merlini, D., preprint.
5. Merlini, D. and Gruber, D., preprint.