JOURNAL OF OPERATOR THEORY, vol.57, no.1, pp.19-34, 2007 (SCI-Expanded)
Let T = {T(t)}(t >= 0) be a continuous semigroup of contractions on a Hilbert space. We define A(T) as the closure of the set {(f) over cap (T) : f is an element of L-1 (R+)} with respect to the operator-norm topology, where (f) over cap (T) =