1. Equivalence of a behavioral distance and the gap metric;Ball;Systems Control Lett.,2006
2. On the space of subspaces of a Banach space;Brown;J. Lond. Math. Soc.,1972
3. On gaps between bounded operators;Cvetcovic;Publ. Inst. Math. (NS),2002
4. Spectral Theory and Differential Operators;Edmunds,1987
5. Functional Analysis. An Introduction;Eidelman,2004