4,328 research outputs found
Vector Reachability Problem in
The decision problems on matrices were intensively studied for many decades
as matrix products play an essential role in the representation of various
computational processes. However, many computational problems for matrix
semigroups are inherently difficult to solve even for problems in low
dimensions and most matrix semigroup problems become undecidable in general
starting from dimension three or four.
This paper solves two open problems about the decidability of the vector
reachability problem over a finitely generated semigroup of matrices from
and the point to point reachability (over rational
numbers) for fractional linear transformations, where associated matrices are
from . The approach to solving reachability problems
is based on the characterization of reachability paths between points which is
followed by the translation of numerical problems on matrices into
computational and combinatorial problems on words and formal languages. We also
give a geometric interpretation of reachability paths and extend the
decidability results to matrix products represented by arbitrary labelled
directed graphs. Finally, we will use this technique to prove that a special
case of the scalar reachability problem is decidable
Computational problems in matrix semigroups
This thesis deals with computational problems that are defined on matrix
semigroups, which playa pivotal role in Mathematics and Computer Science
in such areas as control theory, dynamical systems, hybrid systems, computational
geometry and both classical and quantum computing to name but
a few. Properties that researchers wish to study in such fields often turn out
to be questions regarding the structure of the underlying matrix semigroup
and thus the study of computational problems on such algebraic structures
in linear algebra is of intrinsic importance.
Many natural problems concerning matrix semigroups can be proven
to be intractable or indeed even unsolvable in a formal mathematical sense.
Thus, related problems concerning physical, chemical and biological systems
modelled by such structures have properties which are not amenable to
algorithmic procedures to determine their values.
With such recalcitrant problems we often find that there exists a tight
border between decidability and undecidability dependent upon particular
parameters of the system. Examining this border allows us to determine
which properties we can hope to derive algorithmically and those problems
which will forever be out of our reach, regardless of any future advances in
computational speed.
There are a plethora of open problems in the field related to dynamical
systems, control theory and number theory which we detail throughout
this thesis. We examine undecidability in matrix semigroups for a variety
of different problems such as membership and vector reachability problems,
semigroup intersection emptiness testing and freeness, all of which are well
known from the literature. We also formulate and survey decidability questions
for several new problems such as vector ambiguity, recurrent matrix
problems, the presence of any diagonal matrix and quaternion matrix semigroups,
all of which we feel give a broader perspective to the underlying
structure of matrix semigroups
A minimal nonfinitely based semigroup whose variety is polynomially recognizable
We exhibit a 6-element semigroup that has no finite identity basis but
nevertheless generates a variety whose finite membership problem admits a
polynomial algorithm.Comment: 16 pages, 3 figure
The Identity Correspondence Problem and its Applications
In this paper we study several closely related fundamental problems for words
and matrices. First, we introduce the Identity Correspondence Problem (ICP):
whether a finite set of pairs of words (over a group alphabet) can generate an
identity pair by a sequence of concatenations. We prove that ICP is undecidable
by a reduction of Post's Correspondence Problem via several new encoding
techniques.
In the second part of the paper we use ICP to answer a long standing open
problem concerning matrix semigroups: "Is it decidable for a finitely generated
semigroup S of square integral matrices whether or not the identity matrix
belongs to S?". We show that the problem is undecidable starting from dimension
four even when the number of matrices in the generator is 48. From this fact,
we can immediately derive that the fundamental problem of whether a finite set
of matrices generates a group is also undecidable. We also answer several
question for matrices over different number fields. Apart from the application
to matrix problems, we believe that the Identity Correspondence Problem will
also be useful in identifying new areas of undecidable problems in abstract
algebra, computational questions in logic and combinatorics on words.Comment: We have made some proofs clearer and fixed an important typo from the
published journal version of this article, see footnote 3 on page 1
Parametric Polyhedra with at least Lattice Points: Their Semigroup Structure and the k-Frobenius Problem
Given an integral matrix , the well-studied affine semigroup
\mbox{ Sg} (A)=\{ b : Ax=b, \ x \in {\mathbb Z}^n, x \geq 0\} can be
stratified by the number of lattice points inside the parametric polyhedra
. Such families of parametric polyhedra appear in
many areas of combinatorics, convex geometry, algebra and number theory. The
key themes of this paper are: (1) A structure theory that characterizes
precisely the subset \mbox{ Sg}_{\geq k}(A) of all vectors b \in \mbox{
Sg}(A) such that has at least solutions. We
demonstrate that this set is finitely generated, it is a union of translated
copies of a semigroup which can be computed explicitly via Hilbert bases
computations. Related results can be derived for those right-hand-side vectors
for which has exactly solutions or fewer
than solutions. (2) A computational complexity theory. We show that, when
, are fixed natural numbers, one can compute in polynomial time an
encoding of \mbox{ Sg}_{\geq k}(A) as a multivariate generating function,
using a short sum of rational functions. As a consequence, one can identify all
right-hand-side vectors of bounded norm that have at least solutions. (3)
Applications and computation for the -Frobenius numbers. Using Generating
functions we prove that for fixed the -Frobenius number can be
computed in polynomial time. This generalizes a well-known result for by
R. Kannan. Using some adaptation of dynamic programming we show some practical
computations of -Frobenius numbers and their relatives
Uniform decision problems in automatic semigroups
We consider various decision problems for automatic semigroups, which involve
the provision of an automatic structure as part of the problem instance. With
mild restrictions on the automatic structure, which seem to be necessary to
make the problem well-defined, the uniform word problem for semigroups
described by automatic structures is decidable. Under the same conditions, we
show that one can also decide whether the semigroup is completely simple or
completely zero-simple; in the case that it is, one can compute a Rees matrix
representation for the semigroup, in the form of a Rees matrix together with an
automatic structure for its maximal subgroup. On the other hand, we show that
it is undecidable in general whether a given element of a given automatic
monoid has a right inverse.Comment: 19 page
Reachability problems for systems with linear dynamics
This thesis deals with reachability and freeness problems for systems with linear dynamics, including hybrid systems and matrix semigroups. Hybrid systems are a type of dynamical system that exhibit both continuous and discrete dynamic behaviour. Thus they are particularly useful in modelling practical real world systems which can both flow (continuous behaviour) and jump (discrete behaviour). Decision questions for matrix semigroups have attracted a great deal of attention in both the Mathematics and Theoretical Computer Science communities. They can also be used to model applications with only discrete components.
For a computational model, the reachability problem asks whether we can reach a target point starting from an initial point, which is a natural question both in theoretical study and for real-world applications. By studying this problem and its variations, we shall prove in a formal mathematical sense that many problems are intractable or even unsolvable. Thus we know when such a problem appears in other areas like Biology, Physics or Chemistry, either the problem itself needs to be simplified, or it should by studied by approximation.
In this thesis we concentrate on a specific hybrid system model, called an HPCD, and its variations. The objective of studying this model is twofold: to obtain the most expressive system for which reachability is algorithmically solvable and to explore the simplest system for which it is impossible to solve. For the solvable sub-cases, we shall also study whether reachability is in some sense easy or hard by determining which complexity classes the problem belongs to, such as P, NP(-hard) and PSPACE(-hard). Some undecidable results for matrix semigroups are also shown, which both strengthen our knowledge of the structure of matrix semigroups, and lead to some undecidability results for other models
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