1. Kato, T.,Perturbation Theory for Linear Operators, Springer-Verlag, New York, New York, 1966.
2. Strang, G., andFix, G.,An Analysis of the Finite Element Method, Prentice-Hall, Englewood Cliffs, New Jersey, 1973.
3. Russell, D.,Controllability and Stabilizability Theory for Linear Partial Differential Equations: Recent Progress and Open Questions, University of Wisconsin, Mathematics Research Center, MRC Technical Summary Report No. 1700, 1976.
4. Fattorini, H.,On Complete Controllability of Linear Systems, Journal of Differential Equations, Vol. 3, pp. 391?402, 1967.
5. Slemrod, M.,A Note on Complete Controllability and Stabilizability for Linear Control Systems in Hilbert Space, SIAM Journal of Control, Vol. 12, pp. 500?508, 1974.