Research ArticleOpen accessPhrase in the titleOn a Class of Nonhomogeneous Fields in Hilbert SpaceRaéd HatamlehTwo-parametric semigroups of operators in Hilbert space with bounded infinitesimal doubly commuting operators are studied. The characteristics describing deviation of a semigroup from unitary one, when infinitesimal operators are unitary, in particular, nonunitary index, have been introduced. Necessary and sufficient conditions for nonunitary index finiteness have been obtained. …Journal of Mathematics and Statistics, Vol. 3, Issue 4 · 31 Dec 2007 · 6,682 accessesView PDFCite