1. Theory of Linear Operators in Hilbert Space;Akhiezer,1993
2. The invariance of the index in the metric space of closed operators;Cordes;J. Math. Mech.,1963
3. Functional Analysis;Eidelman,2004
4. The gap of the graph of a matrix;Habibi;Linear Algebra Appl.,1993
5. Basic Classes of Linear Operators;Gohberg,2003