(1) Fachbereich Mathematik, Universität Dortmund, 44221 Dortmund, Germany;(2) School of Mathematics, University of Leeds, Leeds, LS2 9JT, UK;(3) Department of Mathematics, University of York, Heslington, York, YO10 5DD, UK
Abstract:
We derive new necessary and sufficient conditions for admissibility
of observation operators for certain C0-semigroups. We also prove a new
sufficient criterion for admissibility for observation operators with infinite-dimensional
output space on contraction semigroups. If the contraction semigroup
is completely non-unitary and its co-generator has finite defect indices,
then this criterion is also necessary. In the case of the right shift semigroup
on L2(0,), these conditions translate into conditions for the boundedness
of Hankel operators.