1. Bonanzinga V.,Derivazioni integrabili, Tesi di dottorato, 1995.
2. Carfi V.,Integrable derivations in ring of analytic type over a DVR, Atti Sem. Mat. Fis. Università di Modena,32 (1983), No. 1, 1–10.
3. Fossum R. M.,Invariants and formal group Law Actions, in Contemporary Mathematics, vol. 43, 1985.
4. Frolhlich A.,Formal groups, in Lecture Notes in Math., 74, Springer-Verlag, New York/Berlin, 1968.
5. Hazewinkel M.,Formal group and applications, Academic press, New York/San Francisco, 1978.