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In this paper we study hyperbolicty of the universal group $U(P)$ of a pregroup $P$. Given a finitely generated group $G$ and a pregroup $P$ such that $G \simeq U(P)$, we provide a particular set of axioms on $P$ which ensure that $G$ is word-hyperbolic., Comment: 22 pages, 16 figures

In this paper we consider the {\em conjugacy stability} property of subgroups and provide effective procedures to solve the problem in several classes of groups. In particular, we start with free groups, that is, we give an effective procedure to find out if a finitely generated subgroup of a free group is conjugacy stable. Then we further generalize this result to quasi-convex subgroups of torsion-free hyperbolic groups and finitely generated subgroups of limit groups., Comment: 11 pages

In their first article, the authors initiated a systematic study of hyperbolic $\Lambda$-metric spaces, where $\Lambda$ is an ordered abelian group, and groups acting on such spaces. The present paper concentrates on the case $\Lambda = \mathbf{Z}^n$ taken with the right lexicographic order and studies the structure of finitely generated groups acting on hyperbolic $\mathbf{Z}^n$-metric spaces. Under certain constraints, the structure of such groups is described in terms of a {\em hierarchy} similar to the one established for $\mathbf{Z}^n$-free groups by Kharlampovich, Myasnikov, Remeslennikov and Serbin., Comment: 24 pages, 3 figures

In this paper we study group actions on hyperbolic $\Lambda$-metric spaces, where $\Lambda$ is an ordered abelian group. $\Lambda$-metric spaces were first introduced by Morgan and Shalen in their study of hyperbolic structures and then Chiswell, following Gromov's ideas, introduced the notion of hyperbolicty for such spaces. Only the case of 0-hyperbolic $\Lambda$-metric spaces (that is, $\Lambda$-trees) was systematically studied, while the theory of general hyperbolic $\Lambda$-metric spaces was not developed at all. Hence, one of the goals of the present paper was to fill this gap and translate basic notions and results from the theory of group actions on hyperbolic (in the usual sense) spaces to the case of $\Lambda$-metric spaces for an arbitrary $\Lambda$. The other goal was to show some principal difficulties which arise in this generalization and the ways to deal with them., Comment: 61 pages, 6 figures

In this paper we study random walks on a finitely generated group $G$ which has a free action on a $\mathbb{Z}^n$-tree. We show that if $G$ is non-abelian and acts minimally, freely and without inversions on a locally finite $\mathbb{Z}^n$-tree $\Gamma$ with the set of open ends ${\rm Ends}(\Gamma)$, then for every non-degenerate probability measure $\mu$ on $G$ there exists a unique $\mu$-stationary probability measure $\nu_\mu$ on ${\rm Ends}(\Gamma)$, and the space $({\rm Ends}(\Gamma), \nu_\mu)$ is a $\mu$-boundary. Moreover, if $\mu$ has finite first moment with respect to the word metric on $G$ (induced by a finite generating set), then the measure space $({\rm Ends}(\Gamma), \nu_\mu)$ is isomorphic to the Poisson--Furstenberg boundary of $(G, \mu)$., Comment: 29 pages

In this paper we survey recent developments in the theory of groups acting on $\Lambda$-trees. We are trying to unify all significant methods and techniques, both classical and recently developed, in an attempt to present various faces of the theory and to show how these methods can be used to solve major problems about finitely presented $\Lambda$-free groups. Besides surveying results known up to date we draw many new corollaries concerning structural and algorithmic properties of such groups., Comment: 135 pages, 18 figures. arXiv admin note: substantial text overlap with arXiv:0911.0209, arXiv:0907.2356, arXiv:1208.4640, arXiv:1107.0425

In the present paper we continue studying regular free group actions on $\mathbb{Z}^n$-trees. We show that every finitely generated $\mathbb{Z}^n$-free group $G$ can be embedded into a finitely generated $\mathbb{Z}^n$-free group $H$ acting regularly on the underlying $\mathbb{Z}^n$-tree (we call $H$ a {\em regular $\mathbb{Z}^n$-completion} of $G$) so that the action of $G$ is preserved. Moreover, if $G$ is effectively represented as a group of $\mathbb{Z}^n$-words then the construction of $H$ is effective and $H$ is also effectively represented as a group of $\mathbb{Z}^n$-words., Comment: 22 pages

Groups acting freely on Z^n-trees (Z^n-free groups) play a key role in the study of non-archimedean group actions. Following Stallings' ideas, we develop graph-theoretic techniques to investigate subgroup structure of Z^n-free groups. As an immediate application of the presented method, we give an effective solution to the Uniform Membership Problem and the Power Problem in Z^n-free groups., Comment: 41 pages

This is the second paper in a series of three, where we take on the unified theory of non-Archimedean group actions, length functions and infinite words. Here, for an arbitrary group $G$ of infinite words over an ordered abelian group $\Lambda$ we construct a $\Lambda$-tree $\Gamma_G$ equipped with a free action of $G$. Moreover, we show that $\Gamma_G$ is a universal tree for $G$ in the sense that it isometrically embeds in every $\Lambda$-tree equipped with a free $G$-action compatible with the original length function on $G$., Comment: 20 pages, 4 figures

This is the first paper in a series of three where we take on the unified theory of non-Archimedean group actions, length functions and infinite words. Our main goal is to show that group actions on Z^n-trees give one a powerful tool to study groups. All finitely generated groups acting freely on -trees also act freely on some Z^n-trees, but the latter ones form a much larger class. The natural effectiveness of all constructions for $Z^n-actions (which is not the case for R-trees) comes along with a robust algorithmic theory. In this paper we describe the algebraic structure of finitely generated groups acting freely and regularly on Z^n-trees and give necessary and sufficient conditions for such actions., Comment: 40 pages

Using graph-theoretic techniques for f.g. subgroups of $F^{\mathbb{Z}[t]}$ we provide a criterion for a f.g. subgroup of a f.g. fully residually free group to be of finite index. Moreover, we show that this criterion can be checked effectively. Also we obtain an analogue of Greenberg-Stallings Theorem for f.g. fully residually free groups, and prove that a f.g. non-abelian subgroup of a f.g. fully residually free group is of finite index in its commensurator., Comment: 21 pages, 1 figure