1. M. S. Livshits, ?A class of linear operators in Hilbert space,? Mat. Sb.,19 (61), No. 2, 239?260 (1946).
2. A. V. Shtraus, ?Characteristic functions of linear operators,? Izv. Akad. Nauk SSSR,24, 43?74 (1960).
3. A. V. Shtraus, ?A theorem of multiplication of characteristic functions of linear operators,? Dokl. Akad. Nauk SSSR,126, No. 4, 723?726 (1959).
4. A. V. Kuzhel', ?The reduction of unbounded nonself-adjoint operators to triangular form,? Dokl. Akad. Nauk SSSR,119, No. 5, 868?887 (1958).
5. A. V. Kuzhel', ?The spectrum of a regular quasidifferential operator,? Dokl. Akad. Nauk SSSR,156, No. 4, 731?733 (1964).