Differentiability of perturbed semigroups and delay semigroups

Suppose that A generates a Co-semigroup T on a Banach space X. In 1953 R. S. Phillips showed that, for each bounded operator B on X, the perturbation A + B of A generates a Co-semigroup on X, and he considered whether certain classes of semigroups are stable under such perturbations. This study was...