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3. W.M. Wonham, Linear Multivariable Control: a geometric approach, Springer-Verlag, Berlin, 1979.
4. V.I. Korobov and R. Rabakh, Exact controllability in Banach space, Dif. Equations 15 (1979), 1531?1537.
5. G. Eckstein, Exact controllability and spectrum assignment in ?Topics in Modern Operator Theory?, Birkhäuser, Basel, 1981, 81?94.