1. Defect indices of Jm-matrices and of differential operators with polynomial coefficients;Čistjakov;Mat. Sb.,1971
2. Simplicity of linear ordinary differential operators;Gilbert;J. Differential Equations,1972
3. Perturbation Theory for Linear Operators;Kato,1966
4. Infinite J-matrices and the matrix moment problem;Kreǐn;Dokl. Akad. Nauk SSSR,1949
5. Über die aus quadrierbaren Hermiteschen Matrizen entstehenden Operatoren;Nagel;Math. Ann.,1936