arXiv subjects
Publications and source records attributed to Harald K. Wimmer.
We study pairs of finitely generated modules over a principal ideal domain and their corresponding matrix representations. We introduce equivalence relations for such pairs and determine invariants and canonical forms.
Invariant subspaces of a matrix $A$ are considered which are obtained by truncation of a Jordan basis of a generalized eigenspace of $A$. We characterize those subspaces which are independent of the choice of the Jordan basis. An application to Hamilton matrices and algebraic Riccati equations is given.
If the inverse of a nonsingular polynomial matrix $L$ has a polynomial part then one can associate with $L$ a module over the ring of proper rational functions, which is related to the structure of $L$ at infinity. In this paper we characterize homomorphisms of such modules.
Let $V$ be a finite dimensional vector space over a field $K$ and $f$ a $K$-endomorphism of $V$. In this paper we study three types of $f$-invariant subspaces, namely hyperinvariant subspaces, which are invariant under all endomorphisms of $V$ that commute with $f$, characteristic subspaces, which remain fixed under all automorphisms of $V$ that commute with $f$, and marked subspaces, which have a Jordan basis (with respect to $f_{|X}$) that can be extended to a Jordan basis of $V$. We show that a subspace is hyperinvariant if and only if it is characteristic and marked. If $K$ has more than two elements then each characteristic subspace is hyperinvariant.
Let $f$ be an endomorphism of a vector space $V$ over a field $K$. An $f$-invariant subspace $X \subseteq V$ is called hyperinvariant (respectively characteristic) if $X$ is invariant under all endomorphisms (respectively automorphisms) that commute with $f$. If $|K| > 2$ then all characteristic subspaces are hyperinvariant. If $|K| = 2$ then there are endomorphisms $f$ with invariant subspaces that are characteristic but not hyperinvariant. In this paper we give a new proof of a theorem of Shoda, which provides a necessary and sufficient condition for the existence of characteristic non-hyperinvariant subspaces.
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.
Companion matrices of the second type are characterized by properties that involve bilinear maps.
A submodule $W$ of a p-primary module $M$ of bounded order is known to be regular if $W$ and $M$ have simultaneous bases. In this paper we derive necessary and sufficient conditions for regularity of a submodule.
A subspace $X$ of a vector space over a field $K$ is hyperinvariant with respect to an endomorphism $f$ of $V$ if it is invariant for all endomorphisms of $V$ that commute with $f$. We assume that $f$ is locally nilpotent, that is, every $ x \in V $ is annihilated by some power of $f$, and that $V$ is an infinite direct sum of $f$-cyclic subspaces. In this note we describe the lattice of hyperinvariant subspaces of $V$. We extend results of Fillmore, Herrero and Longstaff (Linear Algebra Appl. 17 (1977), 125--132) to infinite dimensional spaces.
If $f$ is an endomorphism of a finite dimensional vector space over a field $K$ then an invariant subspace $X \subseteq V$ is called hyperinvariant (respectively, characteristic) if $X$ is invariant under all endomorphisms (respectively, automorphisms) that commute with $f$. According to Shoda (Math. Zeit. 31, 611--624, 1930) only if $|K| = 2$ then there exist endomorphisms $f$ with invariant subspaces that are characteristic but not hyperinvariant. In this paper we obtain a description of the set of all characteristic non-hyperinvariant subspaces for nilpotent maps $f$ with exactly two unrepeated elementary divisors.
Let $V$ and $W$ be matrices of size $ n \times pk$ and $q m \times n $, respectively. A necessary and sufficient condition is given for the existence of a triple $(A,B,C)$ such that $V$ a $k$-step reachability matrix of $(A,B)$ and $W$ an $m$-step observability matrix of $(A,C)$.