Characteristic subspaces and hyperinvariant frames

Let $f$ be an endomorphism of a finite dimensional vector space $V$ over a field $K$. An $f$-invariant subspace of $V$ is called hyperinvariant (respectively characteristic) if it is invariant under all endomorphisms (respectively automorphisms) that commute with $f$. We assume $|K| = 2$, since all characteristic subspaces are hyperinvariant if $|K| > 2$. The hyperinvariant hull $W^h$ of a subspace $W$ of $V$ is defined to be the smallest hyperinvariant subspace of $V$ that contains $W$, the hyperinvariant kernel $W_H$ of $W$ is the largest hyperinvariant subspace of $V$ that is contained in $W$, and the pair $( W_H, W^h)$ is the hyperinvariant frame of $W$. In this paper we study hyperinvariant frames of characteristic non-hyperinvariant subspaces $W$. We show that all invariant subspaces in the interval $[ W_H, W^h ]$ are characteristic. We use this result for the construction of characteristic non-hyperinvariant subspaces.