1. N. E. Firsova, Zap. Nauchn. Semin. LOMI,51, 183 (1975).
2. N. E. Firsova, ?Trace formula for a perturbed Schrödinger operator with periodic potential,? Diploma Thesis [in Russian], Leningrad State University (1971).
3. N. E. Firsova, Problemy Matem. Fiziki (LGU) No. 7, 162 (1974).
4. N. E. Firsova, Problemy Matem. Fiziki (LGU), No. 8, 158 (1976).
5. M. Sh. Birman, Mat. Sb.,55, 125 (1961).