Hypercyclic semigroups and somewhere dense orbits

We study hypercyclicity of linear strongly continuous semigroups. In the case of iterations of a single operator Bourdon and Feldman have recently proved that the existence of somewhere dense orbits implies hypercyclicity. We show the corresponding result for semigroups. As a consequence, a conjecture of Herrero concerning iterations of a single operator also holds for strongly continuous semigroups. To cite this article: G. Costakis, A. Peris, C. R. Acad. Sci. Paris, Ser. I 335 (2002) 895–898.