253 research outputs found
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
Subgroups of free idempotent generated semigroups: full linear monoid
We develop some new topological tools to study maximal subgroups of free
idempotent generated semigroups. As an application, we show that the rank 1
component of the free idempotent generated semigroup of the biordered set of a
full matrix monoid of n x n matrices, n>2$ over a division ring Q has maximal
subgroup isomorphic to the multiplicative subgroup of Q.Comment: We hope to use similar methods to study the higher rank component
Matrix Semigroup Freeness Problems in SL(2, Z)
In this paper we study decidability and complexity of decision problems on matrices from the special linear group SL(2,Z). In particular, we study the freeness problem: given a finite set of matrices G generating a multiplicative semigroup S, decide whether each element of S has at most one factorization over G. In other words, is G a code? We show that the problem of deciding whether a matrix semigroup in SL(2,Z) is non-free is NP-hard. Then, we study questions about the number of factorizations of matrices in the matrix semigroup such as the finite freeness problem, the recurrent matrix problem, the unique factorizability problem, etc. Finally, we show that some factorization problems could be even harder in SL(2,Z), for example we show that to decide whether every prime matrix has at most k factorizations is PSPACE-hard
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
On the decidability of semigroup freeness
This paper deals with the decidability of semigroup freeness. More precisely,
the freeness problem over a semigroup S is defined as: given a finite subset X
of S, decide whether each element of S has at most one factorization over X. To
date, the decidabilities of two freeness problems have been closely examined.
In 1953, Sardinas and Patterson proposed a now famous algorithm for the
freeness problem over the free monoid. In 1991, Klarner, Birget and Satterfield
proved the undecidability of the freeness problem over three-by-three integer
matrices. Both results led to the publication of many subsequent papers. The
aim of the present paper is three-fold: (i) to present general results
concerning freeness problems, (ii) to study the decidability of freeness
problems over various particular semigroups (special attention is devoted to
multiplicative matrix semigroups), and (iii) to propose precise, challenging
open questions in order to promote the study of the topic.Comment: 46 pages. 1 table. To appear in RAIR
Acceptance Ambiguity for Quantum Automata
We consider notions of freeness and ambiguity for the acceptance probability of Moore-Crutchfield Measure Once Quantum Finite Automata (MO-QFA). We study the distribution of acceptance probabilities of such MO-QFA, which is partly motivated by similar freeness problems for matrix semigroups and other computational models. We show that determining if the acceptance probabilities of all possible input words are unique is undecidable for 32 state MO-QFA, even when all unitary matrices and the projection matrix are rational and the initial configuration is defined over real algebraic numbers. We utilize properties of the skew field of quaternions, free rotation groups, representations of tuples of rationals as a linear sum of radicals and a reduction of the mixed modification Post\u27s correspondence problem
Matrix Semigroup Freeness Problems in
In this paper we study decidability and complexity of decision problems on matrices from the special linear group . In particular, we study the freeness problem: given a finite set of matrices generating a multiplicative semigroup , decide whether each element of has at most one factorization over . In other words, is a code? We show that the problem of deciding whether a matrix semigroup in is non-free is NP-hard. Then, we study questions about the number of factorizations of matrices in the matrix semigroup such as the finite freeness problem, the recurrent matrix problem, the unique factorizability problem, etc. Finally, we show that some factorization problems could be even harder in , for example we show that to decide whether every prime matrix has at most factorizations is PSPACE-hard
On bi-free De Finetti theorems
We investigate possible generalizations of the de Finetti theorem to bi-free
probability. We first introduce a twisted action of the quantum permutation
groups corresponding to the combinatorics of bi-freeness. We then study
properties of families of pairs of variables which are invariant under this
action, both in the bi-noncommutative setting and in the usual noncommutative
setting. We do not have a completely satisfying analogue of the de Finetti
theorem, but we have partial results leading the way. We end with suggestions
concerning the symmetries of a potential notion of n-freeness.Comment: 16 pages. Major rewriting. In the first version the main theorem was
stated through an embedding into a B-B-noncommutative probability space
making it much weaker than what the proof really contains. It has therefore
been split into two independent statements clarifying how far we are able to
extend the de Finetti theorem to the bi-free settin
Composition problems for braids: Membership, Identity and Freeness
In this paper we investigate the decidability and complexity of problems related to braid composition. While all known problems for a class of braids with three strands, , have polynomial time solutions we prove that a very natural question for braid composition, the membership problem, is NP-complete for braids with only three strands. The membership problem is decidable in NP for , but it becomes harder for a class of braids with more strands. In particular we show that fundamental problems about braid compositions are undecidable for braids with at least five strands, but decidability of these problems for remains open. Finally we show that the freeness problem for semigroups of braids from is also decidable in NP. The paper introduces a few challenging algorithmic problems about topological braids opening new connections between braid groups, combinatorics on words, complexity theory and provides solutions for some of these problems by application of several techniques from automata theory, matrix semigroups and algorithms
Vector Reachability Problem in SL(2,Z)
This paper solves three open problems about the decidability of the vector and scalar reachability problems and the point to point reachability by fractional linear transformations over finitely generated semigroups of matrices from . Our approach to solving these problems is based on the characterization of reachability paths between vectors or points, which is then used to translate the numerical problems on matrices into computational problems on words and regular languages. We will also give geometric interpretations of these results
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