  We study semigroups of bounded operators on a Banach space such that the
members of the semigroup are continuous with respect to various weak topologies
and we give sufficient conditions for the generator of the semigroup to be
closed with respect to the topologies involved. The proofs of these results use
the Laplace transforms of the semigroup. Thus we first give sufficient
conditions for Pettis integrability of vector valued functions with respect to
scalar measures.
