“…Recently there has been an increasing interest in the asymptotic behavior of solutions of differential equations in Banach spaces, in particular, in the unbounded case (see, e.g., [2], [3], [7], [8], [9], [13], [16], [22]). In this direction, we would like to mention a recent paper [14] in which a new characterization of exponential dichotomy was given in Hilbert spaces using only conditions on d dt − A(t) and A(t) − d dt (more precisely, its closure).…”