The approximate controllability of fractional linear evolution systems is
considered in this paper. Firstly, the definitions of the mild solution and
the approximate controllability of fractional linear evolution systems are
obtained by using the theory of C?semigroups. Secondly, a new set of
necessary and sufficient conditions are established to examine that linear
system is approximately controllable with the help of symmetric operator.
Moreover, the restricted condition of the state space is weakened by means
of the dual mapping. Finally, as applications, the approximate
controllability of nonlinear evolution systems are derived under the
assumption that the corresponding linear system is approximately
controllable. Our work essentially improves and generalized the
corresponding results which are based on strongly continuous semigroups.