On some groups of automorphisms of physical observables

Given a weakly continuous one-parameter group of automorphisms of aC*-algebra of operators on a Hilbert space we show that it is implementable by a strongly continuous one-parameter group of unitary operators belonging to the weak closure of , provided that a certain condition — akin to the boundedness from below of the spectrum of the generators — is satisfied.On leave from the Istituto di Fisica Teorica, Universitá di Napoli.