# Andrii Dmytryshyn Umeå University, Umeå

Umeå University, Umeå

## Theory of Computation, Algebra, Applied Mathematics

https://www8.cs.umu.se/~andrii

## Publications

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**ABSTRACT:**Developing theory, algorithms, and software tools for analyzing matrix pencils whose matrices have various structures are contemporary research problems. Such matrices are often coming from discretizations of systems of differential-algebraic equations. Therefore preserving the structures in the simulations as well as during the analyses of the mathematical models typically means respecting their physical meanings and may be crucial for the applications. This leads to a fast development of structure-preserving methods in numerical linear algebra along with a growing demand for new theories and tools for the analysis of structured matrix pencils, and in particular, an exploration of their behaviour under perturbations. In many cases, the dynamics and characteristics of the underlying physical system are defined by the canonical structure information, i.e. eigenvalues, their multiplicities and Jordan blocks, as well as left and right minimal indices of the associated matrix pencil. Computing canonical structure information is, nevertheless, an ill-posed problem in the sense that small perturbations in the matrices may drastically change the computed information. One approach to investigate such problems is to use the stratification theory for structured matrix pencils. The development of the theory includes constructing stratification (closure hierarchy) graphs of orbits (and bundles) that provide qualitative information for a deeper understanding of how the characteristics of underlying physical systems can change under small perturbations. In turn, for a given system the stratification graphs provide the possibility to identify more degenerate and more generic nearby systems that may lead to a better system design. We develop the stratification theory for Fiedler linearizations of general matrix polynomials, skew-symmetric matrix pencils and matrix polynomial linearizations, and system pencils associated with generalized state-space systems. The novel contributions also include theory and software for computing codimensions, various versal deformations, properties of matrix pencils and matrix polynomials, and general solutions of matrix equations. In particular, the need of solving matrix equations motivated the investigation of the existence of a solution, advancing into a general result on consistency of systems of coupled Sylvester-type matrix equations and blockdiagonalizations of the associated matrices. -
##### Conference Paper: StratiGraph and the Matrix Canonical Structure Toolbox

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**ABSTRACT:**We present StratiGraph and the Matrix Canonical Structure (MCS) Toolbox for Matlab. StratiGraph is a Java software tool for computing and visualizing closure hierarchy graphs of orbits of matrices, polynomial matrices, and various system pencils. An orbit is a manifold of matrices with the same canonical structure (Jordan, Kronecker, etc.) and the stratification (the closure hierarchy graph) reveals how (small) perturbations in the matrices may change the canonical structure. The MCS Toolbox is a framework with datatype objects for representing canonical structures of matrices, matrix pencils, and system pencils. The toolbox can both be used on its own in Matlab and together with StratiGraph, which enables import and export of canonical structures between Matlab and StratiGraph. Under development are new Matlab routines for computing the staircase form of matrices and matrix pencils. The staircase form reveals the canonical structure of the matrix or matrix pencil. - [Show abstract] [Hide abstract]

**ABSTRACT:**We prove Roth's type theorems for systems of matrix equations including an arbitrary mix of Sylvester and *-Sylvester equations, in which also the transpose or conjugate transpose of the unknown matrices appear. In full generality, we derive consistency conditions by proving that such a system has a solution if and only if the associated set of 2 × 2 block matrix representations of the equations are block diagonalizable by (linked) equivalence transformations. Various applications leading to several particular cases have already been investigated in the literature, some recently and some long ago. Solvability of these cases follow immediately from our general consistency theory. We also show how to apply our main result to systems of Stein-type matrix equations. - [Show abstract] [Hide abstract]

**ABSTRACT:**We study how elementary divisors and minimal indices of a skew-symmetric matrix polynomial of odd degree may change under small perturbations of the matrix coefficients. We investigate these changes qualitatively by constructing the stratifications (closure hierarchy graphs) of orbits and bundles for skew-symmetric linearizations. We also derive the necessary and sufficient conditions for the existence of a skew-symmetric matrix polynomial with prescribed degree, elementary divisors, and minimal indices. - [Show abstract] [Hide abstract]

**ABSTRACT:**We investigate the changes under small perturbations of the canonical structure information for a system pencil A B C D − s E 0 0 0 , det(E) ≠ 0, associated with a (generalized) linear time-invariant state-space system. The equivalence class of the pencil is taken with respect to feedback-injection equivalence transformation. The results allow to track possible changes under small perturbations of important linear system characteristics. - [Show abstract] [Hide abstract]

**ABSTRACT:**We study how small perturbations of matrix polynomials may change their elementary divisors and minimal indices by constructing the closure hierarchy graphs (stratifications) of orbits and bundles of matrix polynomial Fiedler linearizations. We show that the stratifica-tion graphs do not depend on the choice of Fiedler linearization which means that all the spaces of the matrix polynomial Fiedler lineariza-tions have the same geometry (topology). The results are illustrated by examples using the software tool StratiGraph. - [Show abstract] [Hide abstract]

**ABSTRACT:**We construct the Hasse diagrams for the closure ordering on the sets of congruence classes of 2x2 and 3x3 matrices. In other words, we construct two directed graphs whose vertices are 2x2 or, respectively, 3x3 canonical matrices for congruence and there is a directed path from A to B if and only if A can be transformed by an arbitrarily small perturbation to a matrix that is congruent to B. It is important to know all such matrices B if A is known only approximately. A bundle of matrices under congruence is defined as a set of square matrices A for which the pencils A+xA^T are strictly equivalent. We give motivations of this definition and construct two Hasse diagrams for the closure ordering on the sets of bundles of 2x2 and, respectively, 3x3 matrices under congruence. - [Show abstract] [Hide abstract]

**ABSTRACT:**We study how small perturbations of a skew-symmetric matrix pencil may change its canonical form under congruence. This problem is also known as the stratification problem of skew-symmetric matrix pencil orbits and bundles. In other words, we investigate when the closure of the congruence orbit (or bundle) of a skew-symmetric matrix pencil contains the congruence orbit (or bundle) of another skew-symmetric matrix pencil. The developed theory relies on our main theorem stating that a skew-symmetric matrix pencil $A-\lambda B$ can be approximated by pencils strictly equivalent to a skew-symmetric matrix pencil $C-\lambda D$ if and only if $A-\lambda B$ can be approximated by pencils congruent to $C-\lambda D$. Read More: http://epubs.siam.org/doi/abs/10.1137/140956841 - [Show abstract] [Hide abstract]

**ABSTRACT:**V.I. Arnold [Russian Math. Surveys 26(2) (1971) 29-43] constructed a miniversal deformation of a square complex matrix under similarity; that is, a simple normal form to which not only a given square matrix A but all matrices B close to it can be reduced by similarity transformations that smoothly depend on the entries of B. We give miniversal deformations of matrices of sesquilinear forms; that is, of square complex matrices under *congruence, and construct an analytic reducing transformation to a miniversal deformation. Analogous results for matrices under congruence were obtained by the authors in [Linear Algebra Appl. 436 (2012) 2670-2700]. - [Show abstract] [Hide abstract]

**ABSTRACT:**Investigating the properties, explaining, and predicting the behaviour of a physical system described by a system (matrix) pencil often require the understanding of how canonical structure information of the system pencil may change, e.g., how eigenvalues coalesce or split apart, due to perturbations in the matrix pencil elements. Often these system pencils have different block-partitioning and / or symmetries. We study changes of the congruence canonical form of a complex skew-symmetric matrix pencil under small perturbations. The problem of computing the congruence canonical form is known to be ill-posed: both the canonical form and the reduction transformation depend discontinuously on the entries of a pencil. Thus it is important to know the canonical forms of all such pencils that are close to the investigated pencil. One way to investigate this problem is to construct the stratification of orbits and bundles of the pencils. To be precise, for any problem dimension we construct the closure hierarchy graph for congruence orbits or bundles. Each node (vertex) of the graph represents an orbit (or a bundle) and each edge represents the cover/closure relation. Such a relation means that there is a path from one node to another node if and only if a skew-symmetric matrix pencil corresponding to the first node can be transformed by an arbitrarily small perturbation to a skew-symmetric matrix pencil corresponding to the second node. From the graph it is straightforward to identify more degenerate and more generic nearby canonical structures. A necessary (but not sufficient) condition for one orbit being in the closure of another is that the first orbit has larger codimension than the second one. Therefore we compute the codimensions of the congruence orbits (or bundles). It is done via the solutions of an associated homogeneous system of matrix equations. The complete stratification is done by proving the relation between equivalence and congruence for the skew-symmetric matrix pencils. This relation allows us to use the known result about the stratifications of general matrix pencils (under strict equivalence) in order to stratify skew-symmetric matrix pencils under congruence. Matlab functions to work with skew-symmetric matrix pencils and a number of other types of symmetries for matrices and matrix pencils are developed and included in the Matrix Canonical Structure (MCS) Toolbox. -
##### Article: Symmetric matrix pencils: Codimension counts and the solution of a pair of matrix equations

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**ABSTRACT:**The set of all solutions to the homogeneous system of matrix equations (X^TA + AX, X^TB + BX) = (0, 0), where (A,B) is a pair of symmetric matrices of the same size, is characterised. In addition, the codimension of the orbit of (A,B) under congruence is calculated. This paper is a natural continuation of the article [A. Dmytryshyn, B. Kågström, and V.V. Sergeichuk. Skew-symmetric matrix pencils: Codimension counts and the solution of a pair of matrix equations. Linear Algebra Appl., 438:3375–3396, 2013.], where the corresponding problems for skew-symmetric matrix pencils are solved. The new results will be useful in the development of the stratification theory for orbits of symmetric matrix pencils. - [Show abstract] [Hide abstract]

**ABSTRACT:**Matlab functions to work with the canonical structures for congru-ence and *congruence of matrices, and for congruence of symmetric and skew-symmetric matrix pencils are presented. A user can provide the canonical structure objects or create (random) matrix example setups with a desired canonical information, and compute the codi-mensions of the corresponding orbits: if the structural information (the canonical form) of a matrix or a matrix pencil is known it is used for the codimension computations, otherwise they are computed numerically. Some auxiliary functions are provided too. All these functions extend the Matrix Canonical Structure Toolbox. -
##### Article: Skew-symmetric matrix pencils: Codimension counts and the solution of a pair of matrix equations

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**ABSTRACT:**The homogeneous system of matrix equations (XTA+AX,X TB+BX)=(0,0), where (A,B) is a pair of skew-symmetric matrices of the same size is considered: we establish the general solution and calculate the codimension of the orbit of (A,B) under congruence. These results will be useful in the development of the stratification theory for orbits of skew-symmetric matrix pencils. - [Show abstract] [Hide abstract]

**ABSTRACT:**We give a miniversal deformation of each pair of skew-symmetric matrices $(A,B)$ under congruence; that is, a normal form with minimal number of independent parameters to which all matrix pairs $(A+E,B+E')$ close to $(A,B)$ can be reduced by congruence transformation $ (A+E,B+E')\mapsto {\cal S}(E,E')^T (A+E,B+E') {\cal S}(E,E'), {\cal S}(0,0)=I, $ in which ${\cal S}(E,E')$ smoothly depends on the entries of $E$ and $E'$. - [Show abstract] [Hide abstract]

**ABSTRACT:**V.I. Arnold [Russian Math. Surveys 26 (2) (1971) 29-43] constructed a miniversal deformation of matrices under similarity; that is, a simple normal form to which not only a given square matrix A but all matrices B close to it can be reduced by similarity transformations that smoothly depend on the entries of B. We construct a miniversal deformation of matrices under congruence. - [Show abstract] [Hide abstract]

**ABSTRACT:**Using miniversal deformations of a skew-symmetric matrix, a generalization of the Darboux theorem from symplectic geometry is derived. - [Show abstract] [Hide abstract]

**ABSTRACT:**Let F be a field of characteristic different from 2. It is shown that the problems of classifying (i) local commutative associative algebras over F with zero cube radical, (ii) Lie algebras over F with central commutator subalgebra of dimension 3, and (iii) finite p-groups of exponent p with central commutator subgroup of order p(3) are hopeless since each of them contains the problem of classifying symmetric bilinear mappings U x U -> V, or the problem of classifying skew- symmetric bilinear mappings U x U -> V in which U and V are vector spaces over F (consisting of p elements for p-groups (iii)) and V is 3-dimensional. The latter two problems are hopeless since they are wild; i.e., each of them contains the problem of classifying pairs of matrices over F up to similarity. - [Show abstract] [Hide abstract]

**ABSTRACT:**Let F be a ﬁeld of characteristic different from 2. It is shown that the problems of classifying (i) local commutative associative algebras over F with zero cube radical, (ii) Lie algebras over F with central commutator subalgebra of dimension 3, and (iii) ﬁnite p-groups of exponent p with central commutator subgroup of order p^3 are hopeless since each of them contains • the problem of classifying symmetric bilinear mappings U × U → V , or • the problem of classifying skew-symmetric bilinear mappings U × U → V , in which U and V are vector spaces over F (consisting of p elements for p-groups (iii)) and V is 3-dimensional. The latter two problems are hopeless since they are wild; i.e., each of them contains the problem of classifying pairs of matrices over F up to similarity.

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