Jingbo, Wu, Normal Extensions of Semigroups of Operators to Krein Spaces, Acta Mathematica Sinica, New Series Chinese Journal of Mathematics, 7(3), 1991, pp. 205-215
In this paper, we prove that every strongly continuous semigroup of bounded opera tors on a Hilbert space may be extended to a strongly continuous semigroup of normal operators on a larger Krein space. Several equivalent formulations for the case where the extension space is a Pontjagin space are given.