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Perturbation theory of boundary value problems and approximate controllability of perturbed boundary control problems

A semigroup approach for the well-posedness of perturbed nonhomogeneous abstract boundary value problems is developed in this paper. This allows us to introduce a useful variation of constant formula for the solutions. Drawing from this formula, necessary and sufficient conditions for the approximate controllability of such systems are obtained, using the feedback theory of well-posed and regular linear systems developed by Salamon, Staffans and Weiss.