CONTINUITY OF RESTRICTIONS OF $(a, k)$-REGULARIZED RESOLVENT FAMILIES TO INVARIANT SUBSPACES

Abstract

Let $X$ be a Banach space which is continuously embedded in another Banach space $Y$ and is an invariant subspace for an $(a,k)$-regularized resolvent family $R(\cdot)$ of operators on $Y$. It is shown that the restriction of $R(\cdot)$ to $X$ is strongly continuous with respect to the norm of $X$ if and only if all its partial orbits are relatively weakly compact in $X$. This property is shared by many particular cases of $(a,k)$-regularized resolvent families, such as integrated solution families, integrated semigroups, and integrated cosine functions.