Nearly invariant subspaces for shift semigroups

Abstract: Let ${T(t)}{t\geq0}$ be a $C_0$-semigroup on an infinite dimensional separable Hilbert space; a suitable definition of near ${T(t)^*}{t\geq0}$ invariance of a subspace is presented in this paper. A series of prototypical examples for minimal nearly ${S(t)^*}_{t\geq0}$ invariant subspaces for the shift semigroup ${S(t)}{t\geq0}$ on $L^2(0,\infty)$ are demonstrated, which have close links with nearly $T{\theta}^*$ invariance on Hardy spaces of the unit disk for Toeplitz operators associated with an inner function $\theta$. 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.