Publications (38)10.61 Total impact
 Acta Applicandae Mathematicae 10/2014; 133(1):187234. · 0.70 Impact Factor
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ABSTRACT: This paper addresses systems of linear functional equations from an algebraic point of view. We give an introduction to and an overview of recent work by a small group of people including the author of this article on effective methods which determine structural properties of such systems. We focus on parametrizability of the behavior, i.e., the set of solutions in an appropriate signal space, which is equivalent to controllability in many controltheoretic situations. Flatness of the linear system corresponds to the existence of an injective parametrization. Using an algebraic analysis approach, we associate with a linear system a module over a ring of operators. For systems of linear partial differential equations we choose a ring of differential operators, for multidimensional discrete linear systems a ring of shift operators, for linear differential timedelay systems a combination of those, etc. Rings of these kinds are Ore algebras, which admit Janet basis or Gröbner basis computations. Module theory and homological algebra can then be applied effectively to study a linear system via its system module, the interpretation depending on the duality between equations and solutions. In particular, the problem of computing bases of finitely generated free modules (i.e., of computing flat outputs for linear systems) is addressed for different kinds of algebras of operators, e.g., the Weyl algebras. Some work on computer algebra packages, which have been developed in this context, is summarized.Multidimensional Systems and Signal Processing 01/2014; 26(2). · 1.58 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: This paper presents an algorithmic method to study structural properties of nonlinear control systems in dependence of parameters. The result consists of a description of parameter configurations which cause different controltheoretic behaviour of the system (in terms of observability, flatness, etc.). The constructive symbolic method is based on the differential Thomas decomposition into disjoint simple systems, in particular its elimination properties.12/2012;  [Show abstract] [Hide abstract]
ABSTRACT: To compute difference Groebner bases of ideals generated by linear polynomials we adopt to difference polynomial rings the involutive algorithm based on Janetlike division. The algorithm has been implemented in Maple in the form of the package LDA (Linear Difference Algebra) and we describe the main features of the package. Its applications are illustrated by generation of finite difference approximations to linear partial differential equations and by reduction of Feynman integrals. We also present the algorithm for an ideal generated by a finite set of nonlinear difference polynomials. If the algorithm terminates, then it constructs a Groebner basis of the ideal.06/2012;  [Show abstract] [Hide abstract]
ABSTRACT: In this paper, we consider systems of algebraic and nonlinear partial differential equations and inequations. We decompose these systems into socalled simple subsystems and thereby partition the set of solutions. For algebraic systems, simplicity means triangularity, squarefreeness and nonvanishing initials. Differential simplicity extends algebraic simplicity with involutivity. We build upon the constructive ideas of J. M. Thomas and develop them into a new algorithm for disjoint decomposition. The given paper is a revised version of a previous paper and includes the proofs of correctness and termination of our decomposition algorithm. In addition, we illustrate the algorithm with further instructive examples and describe its Maple implementation together with an experimental comparison to some other triangular decomposition algorithms.Journal of Symbolic Computation 08/2011; · 0.71 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Some changes of the traditional scheme for finding rational solutions of linear differential, difference and qdifference homogeneous equations with rational coefficients are proposed. In many cases these changes allow one to predict the absence ...ACM Communications in Computer Algebra 01/2011;  [Show abstract] [Hide abstract]
ABSTRACT: This paper provides methods to decide whether a given analytic function of several complex variables is a linear combination of nitely many given analytic functions with coecients of the following special form: Each one of these coe cients is a composition of an unknown analytic function of less arguments than the function to be expressed, with xed analytic functions. Methods which compute suitable coecient functions in the armative case are presented as well.Mathematics in Computer Science 09/2010; 4:231242.  [Show abstract] [Hide abstract]
ABSTRACT: In this paper we consider disjoint decomposition of algebraic and nonlinear partial differential systems of equations and inequations into socalled simple subsystems. We exploit Thomas decomposition ideas and develop them into a new algorithm. For algebraic systems simplicity means triangularity, squarefreeness and nonvanishing initials. For differential systems the algorithm provides not only algebraic simplicity but also involutivity. The algorithm has been implemented in Maple.08/2010; 
Conference Paper: Thomas Decomposition of Algebraic and Differential Systems.
Computer Algebra in Scientific Computing  12th International Workshop, CASC 2010, Tsakhkadzor, Armenia, September 612, 2010. Proceedings; 01/2010 
Conference Paper: Consistency of finite difference approximations for linear PDE systems and its algorithmic verification.
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ABSTRACT: In this paper we consider finite difference approximations for numerical solving of systems of partial differential equations of the form f1 = · · · = fp = 0, where F := {f1, ..., fp} is a set of linear partial differential polynomials over the field of rational functions with rational coefficients. For orthogonal and uniform solution grids we strengthen the generally accepted concept of equationwise consistency (econsistency) of the difference equations f1 = · · · = fp = 0 as approximation of the differential ones. Instead, we introduce a notion of consistency of the set of all linear consequences of the difference polynomial set f := {f, ..., fp} with the linear subset of the differential ideal 〈F〉. The last consistency, which we call sconsistency (strong consistency), admits algorithmic verification via a Gröbner basis of the difference ideal 〈f〉. Some related illustrative examples of finite difference approximations, including those which are econsistent and sinconsistent, are given.Symbolic and Algebraic Computation, International Symposium, ISSAC 2010, Munich, Germany, July 2528, 2010, Proceedings; 01/2010 
Article: The average number of cycles
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ABSTRACT: The paper remarks on the average number of cycles of the elements of a finite permutation group in general and computes this number for the alternating groups and wreath products using the cycle number indicator.Archiv der Mathematik 11/2009; 93(5):445449. · 0.48 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: This paper explains the relevance of partitioning the set of standard monomials into cones for constructing a Noether normalization for an ideal in a polynomial ring. Such a decomposition of the complement of the corresponding initial ideal in the set of all monomials  also known as a Stanley decomposition  is constructed in the context of Janet bases, in order to come up with sparse coordinate changes which achieve Noether normal position for the given ideal.Journal of Symbolic Computation 10/2009; 44:13591373. · 0.71 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In this work we announce the package to compute connection and Cconnection matrices. is based on our abstract homological algebra package . We emphasize that the notion of braids is irrelevant for the definition and for the computation of such matrices. We introduce the notion of triangles that suffices to state the definition of (C) connection matrices. The notion of octahedra, which is equivalent to that of braids is also introduced.Journal of Symbolic Computation 05/2009; 44(5):540557. · 0.39 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Computing the relations for the coefficients satisfied by the characteristic polynomial of the Kronecker product of a general $n \times n$ matrix by a general $m \times m$ matrix leads to an elimination problem that is already difficult for small values of $n$ and $m$. In this article we focus on the problems for $(n, m) \in \{ (2,3), (2,4), (3,3)$ and use these problems for developing and testing a new elimination technique called elimination by degree steering.Experimental Mathematics 01/2008; 17(2008). · 1.00 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A wellknown result due to J.T. Stafford asserts that a stably free left module M over the Weyl algebras D=An(k) or Bn(k)–where k is a field of characteristic 0–with is free. The purpose of this paper is to present a new constructive proof of this result as well as an effective algorithm for the computation of bases of M. This algorithm, based on the new constructive proofs [Hillebrand, A., Schmale, W., 2001. Towards an effective version of a theorem of Stafford. J. Symbolic Comput. 32, 699–716; Leykin, A., 2004. Algorithmic proofs of two theorems of Stafford. J. Symbolic Comput. 38, 1535–1550] of J.T. Stafford’s result on the number of generators of left ideals of D, performs Gaussian elimination on the formal adjoint of the presentation matrix of M. We show that J.T. Stafford’s result is a particular case of a more general one asserting that a stably free left Dmodule M with is free, where denotes the stable rank of a ring D. This result is constructive if the stability of unimodular vectors with entries in D can be tested. Finally, an algorithm which computes the left projective dimension of a general left Dmodule M defined by means of a finite free resolution is presented. It allows us to check whether or not the left Dmodule M is stably free.Journal of Symbolic Computation 11/2007; 42(111242):11131141. · 0.71 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In the seventies, the study of transfer matrices of timeinvariant linear systems of ordinary differential equations (ODEs) led to the development of the polynomial approach [20, 22, 44]. In particular, the univariate polynomial matrices play a central role in this approach (e.g., Hermite, Smith and Popov forms, invariant factors, primeness, Bézout/Diophantine equations).04/2007: pages 233264;  [Show abstract] [Hide abstract]
ABSTRACT: The central notion of this work is that of a functor between categories of finitely presented modules over socalled computable rings, i.e. rings R where one can algorithmically solve inhomogeneous linear equations with coefficients in R. The paper describes a way allowing one to realize such functors, e.g. Hom, tensor product, Ext, Tor, as a mathematical object in a computer algebra system. Once this is achieved, one can compose and derive functors and even iterate this process without the need of any specific knowledge of these functors. These ideas are realized in the ring independent package homalg. It is designed to extend any computer algebra software implementing the arithmetics of a computable ring R, as soon as the latter contains algorithms to solve inhomogeneous linear equations with coefficients in R. Beside explaining how this suffices, the paper describes the nature of the extensions provided by homalg.Journal of Algebra and Its Applications 01/2007; · 0.37 Impact Factor 
Conference Paper: Some Elimination Problems for Matrices.
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ABSTRACT: New elimination methods are applied to compute polynomial relations for the coefficients of the characteristic polynomial of certain families of matrices such as tensor squares.Computer Algebra in Scientific Computing, 10th International Workshop, CASC 2007, Bonn, Germany, September 1620, 2007, Proceedings; 01/2007  [Show abstract] [Hide abstract]
ABSTRACT: Characteristic zero representations of the (2,3,7)triangle group in degrees up to seven are constructed by using Janet's algorithm for solving polynomial equations. These are used to find families of Hurwitz groups, i.e. finite epimorphic images. For some varieties of representations it is investigated whether additional relations can be uniformly imposed and still result in subvarieties of representations. The methods are of more general interest. Some remarks about the interaction of positive characteristics and characteristic zero are made.Journal of Algebra 06/2006; · 0.60 Impact Factor 
Conference Paper: On the blowingup of stably free behaviours
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ABSTRACT: It is wellknown that a timevarying controllable ordinary differential linear system is flat outside some singularities. In this paper, we prove that every timevarying controllable linear system is a projection of a flat system. We give an explicit description of a flat system which projects onto a given controllable one. This phenomenon is similar to a classical one largely studied in algebraic geometry and called the blowingup of a singularity. These results simplify the ones obtained in [6] and generalize them to MIMO multidimensional systems. Finally, we prove that every controllable multiinput ordinary differential linear system with polynomial coefficients is flat.Decision and Control, 2005 and 2005 European Control Conference. CDCECC '05. 44th IEEE Conference on; 01/2006
Publication Stats
331  Citations  
10.61  Total Impact Points  
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Institutions

2007–2011

RWTH Aachen University
 Lehrstuhl B für Mathematik
Aachen, North RhineWestphalia, Germany
