Nearly invariant subspaces for shift semigroups

Let {T(t)}t⩾0 be a C0-semigroup on an infinite-dimensional separable Hilbert space; a suitable definition of near {T(t)*}t⩾0 invariance of a subspace is presented in this paper. A series of prototypical examples for minimal nearly {S(t)*}t⩾0 invariant subspaces for the shift semigroup {S(t)}t⩾0 on L2(0, ∞) are demonstrated, which have close links with near T∗θ

invariance on Hardy spaces of the unit disk for an inner function θ. Especially, the corresponding subspaces on Hardy spaces of the right half-plane and the unit disk are related to model spaces. This work further includes a discussion on the structure of the closure of certain subspaces related to model spaces in Hardy spaces.