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Abstract: We examine computational problems on quaternion matrix and rotation semigroups. It is shown that in the ultimate case of quaternion matrices, in which multiplication is still associative, most of the decision problems for matrix semigroups are undecidable in dimension two. The geometric interpretation of matrix problems over quaternions is presented in terms of rotation problems for the 2- and 3-sphere. In particular, we show that the reachability of the rotation problem is undecidable on the 3-sphere and other rotation problems can be formulated as matrix problems over complex and hypercomplex numbers. [Copyright &y& Elsevier]

Abstract: In this paper we consider several reachability problems such as vector reachability, membership in matrix semigroups and reachability problems in piecewise linear maps. Since all of these questions are undecidable in general, we work on lowering the bounds for undecidability. In particular, we show an elementary proof of undecidability of the reachability problem for a set of 5 two-dimensional affine transformations. Then, using a modified version of a standard technique, we also prove that the vector reachability problem is undecidable for two (rational) matrices in dimension 11. The above result can be used to show that the system of piecewise linear functions of dimension 12 with only two intervals has an undecidable set-to-point reachability problem. We also show that the “zero in the upper right corner” problem is undecidable for two integral matrices of dimension 18 lowering the bound from 23. [Copyright &y& Elsevier]

Abstract: In this paper, we consider decidability questions that are related to the membership problem in matrix semigroups. In particular, we consider the membership of a given invertible diagonal matrix in a matrix semigroup and then a scalar matrix, which has a separate geometric interpretation. Both problems have been open for any dimensions and are shown to be undecidable in dimension 4 with integral matrices by a reduction of the Post Correspondence Problem (PCP). Although the idea of PCP reduction is standard for such problems, we suggest a new coding technique to cover the case of diagonal matrices. [Copyright &y& Elsevier]

We show that the problem of determining if the identity matrix belongs to a finitely generated semigroup of 2 × 2 matrices from the General Linear Group GL 2 (Z) is solvable in NP. We extend this to prove that the membership problem is decidable in NP for GL 2 (Z) and for any arbitrary regular expression over matrices from the Special Linear group SL 2 (Z). We show that determining if a given finite set of matrices from SL 2 (Z) or the modular group PSL 2 (Z) generates a group or a free semigroup are decidable in NP. Previous algorithms, shown in 2005 by Choffrut and Karhumäki, were in EXPSPACE. Our algorithm is based on new techniques allowing us to operate on compressed word representations of matrices without explicit expansions. When combined with known NP -hard lower bounds, this proves that the membership problem over GL 2 (Z) is NP -complete, and the group problem and the non-freeness problem in SL 2 (Z) are NP -complete. 1 [ABSTRACT FROM AUTHOR]