ACKNOWLEDGMENTS
The author would like to express his gratitude to A. P. Pozhidaev for the suggestion of this topic and the many valuable scientific discussions which have been essential to achieve the main results of this paper. A special acknowledgment to F. Silva Leite for her helpful suggestions while preparing this manuscript.
The results of this paper were presented in 19.07.2000 at the Shirshov seminar on ring theory at the Institute of Mathematics of SB RAS. Work supported in part by ISR and Project Mathematical Physics (under Praxis XXI Program), while the author was visiting the Institute of Mathematics of SB RAS.
Notes
This lemma was suggested by A.P. Pozhidaev.
(observe that if , then
and
can also be represented this way).