1. R. Bott, Enseigh. Math.,23, Nos. 3?4, 209?220 (1977).
2. I. M. Gel'fand and D. B. Fuks, Funkts. Anal. Prilozhen.,2, No. 4, 92?93 (1968).
3. A. A. Kirillov, Memoirs of the Fourth Bulgarian School on Mathematical Physics [in Russian] (1980).
4. V. F. Lazutkin and T. F. Pankratova, Funkts. Anal. Prilozhen.,9, No. 4, 41?48 (1975).
5. A. A. Kirillov, Usp. Mat. Nauk,31, No. 4, 57?76 (1976).