1,981 research outputs found
Index Reduction for Differential-Algebraic Equations with Mixed Matrices
Differential-algebraic equations (DAEs) are widely used for modeling of
dynamical systems. The difficulty in solving numerically a DAE is measured by
its differentiation index. For highly accurate simulation of dynamical systems,
it is important to convert high-index DAEs into low-index DAEs. Most of
existing simulation software packages for dynamical systems are equipped with
an index-reduction algorithm given by Mattsson and S\"{o}derlind.
Unfortunately, this algorithm fails if there are numerical cancellations.
These numerical cancellations are often caused by accurate constants in
structural equations. Distinguishing those accurate constants from generic
parameters that represent physical quantities, Murota and Iri introduced the
notion of a mixed matrix as a mathematical tool for faithful model description
in structural approach to systems analysis. For DAEs described with the use of
mixed matrices, efficient algorithms to compute the index have been developed
by exploiting matroid theory.
This paper presents an index-reduction algorithm for linear DAEs whose
coefficient matrices are mixed matrices, i.e., linear DAEs containing physical
quantities as parameters. Our algorithm detects numerical cancellations between
accurate constants, and transforms a DAE into an equivalent DAE to which
Mattsson--S\"{o}derlind's index-reduction algorithm is applicable. Our
algorithm is based on the combinatorial relaxation approach, which is a
framework to solve a linear algebraic problem by iteratively relaxing it into
an efficiently solvable combinatorial optimization problem. The algorithm does
not rely on symbolic manipulations but on fast combinatorial algorithms on
graphs and matroids. Furthermore, we provide an improved algorithm under an
assumption based on dimensional analysis of dynamical systems.Comment: A preliminary version of this paper is to appear in Proceedings of
the Eighth SIAM Workshop on Combinatorial Scientific Computing, Bergen,
Norway, June 201
On the Reduction of Singularly-Perturbed Linear Differential Systems
In this article, we recover singularly-perturbed linear differential systems
from their turning points and reduce the rank of the singularity in the
parameter to its minimal integer value. Our treatment is Moser-based; that is
to say it is based on the reduction criterion introduced for linear singular
differential systems by Moser. Such algorithms have proved their utility in the
symbolic resolution of the systems of linear functional equations, giving rise
to the package ISOLDE, as well as in the perturbed algebraic eigenvalue
problem. Our algorithm, implemented in the computer algebra system Maple, paves
the way for efficient symbolic resolution of singularly-perturbed linear
differential systems as well as further applications of Moser-based reduction
over bivariate (differential) fields.Comment: Keywords: Moser-based Reduction, Perturbed linear Differential
systems, turning points, Computer algebr
Non-unimodular reductions and N = 4 gauged supergravities
We analyze the class of four-dimensional N = 4 supergravities obtained by
gauging the axionic shift and axionic rescaling symmetries. These theories are
formulated with the machinery of embedding tensors and shown to be deducible
from higher dimensions using a Scherk--Schwarz reduction with a twist by a
non-compact symmetry. This allows to evade the usual unimodularity requirement
and completes the dictionary between heterotic gaugings and fluxes, at least
for the "geometric sector".Comment: 15 page
Duality Twists on a Group Manifold
We study duality-twisted dimensional reductions on a group manifold G, where
the twist is in a group \tilde{G} and examine the conditions for consistency.
We find that if the duality twist is introduced through a group element
\tilde{g} in \tilde{G}, then the flat \tilde{G}-connection A =\tilde{g}^{-1}
d\tilde{g} must have constant components M_n with respect to the basis 1-forms
on G, so that the dependence on the internal coordinates cancels out in the
lower dimensional theory. This condition can be satisfied if and only if M_n
forms a representation of the Lie algebra of G, which then ensures that the
lower dimensional gauge algebra closes. We find the form of this gauge algebra
and compare it to that arising from flux compactifications on twisted tori. As
an example of our construction, we find a new five dimensional gauged, massive
supergravity theory by dimensionally reducing the eight dimensional Type II
supergravity on a three dimensional unimodular, non-semi-simple, non-abelian
group manifold with an SL(3,R) twist.Comment: 22 page
Poisson sigma model on the sphere
We evaluate the path integral of the Poisson sigma model on sphere and study
the correlators of quantum observables. We argue that for the path integral to
be well-defined the corresponding
Poisson structure should be unimodular. The construction of the finite
dimensional BV theory is presented and we argue that it is responsible for the
leading semiclassical contribution. For a (twisted) generalized Kahler manifold
we discuss the gauge fixed action for the Poisson sigma model. Using the
localization we prove that for the holomorphic Poisson structure the
semiclassical result for the correlators is indeed the full quantum result.Comment: 38 page
Generators of split extensions of Abelian groups by cyclic groups
Let be an -generator group with Abelian and
cyclic. We study the Nielsen equivalence classes and T-systems of generating
-tuples of . The subgroup can be turned into a finitely generated
faithful module over a suitable quotient of the integral group ring of .
When is infinite, we show that the Nielsen equivalence classes of the
generating -tuples of correspond bijectively to the orbits of unimodular
rows in under the action of a subgroup of . Making no
assumption on the cardinality of , we exhibit a complete invariant of
Nielsen equivalence in the case . As an application, we classify
Nielsen equivalence classes and T-systems of soluble Baumslag-Solitar groups,
lamplighter groups and split metacyclic groups.Comment: 36 pages, The former Theorem F.ii has been retracted because the
proof was wrong and couldn't be repaired. To appear in Groups, Geometry and
Dynamic
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