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Search Results: 1 - 10 of 130863 matches for " Volodymyr V. Nekrashevych "
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Iterated Monodromy Groups of Quadratic Polynomials, I
Laurent Bartholdi,Volodymyr V. Nekrashevych
Mathematics , 2006,
Abstract: We describe the iterated monodromy groups associated with post-critically finite quadratic polynomials, and explicit their connection to the `kneading sequence' of the polynomial. We then give recursive presentations by generators and relations for these groups, and study some of their properties, like torsion and `branchness'.
On amenability of automata groups
Laurent Bartholdi,Vadim A. Kaimanovich,Volodymyr V. Nekrashevych
Mathematics , 2008, DOI: 10.1215/00127094-2010-046
Abstract: We show that the group of bounded automatic automorphisms of a rooted tree is amenable, which implies amenability of numerous classes of groups generated by finite automata. The proof is based on reducing the problem to showing amenability just of a certain explicit family of groups ("Mother groups") which is done by analyzing the asymptotic properties of random walks on these groups.
From fractal groups to fractal sets
Laurent Bartholdi,Rostislav I. Grigorchuk,Volodymyr V. Nekrashevych
Mathematics , 2002,
Abstract: This paper is a survey, with few proofs, of ideas and notions related to self-similarity of groups, semi-groups and their actions. It attempts to relate these concepts to more familiar ones, such as fractals, self-similar sets, and renormalizable dynamical systems. In particular, it presents a plausible definition of what a "fractal group" should be, and gives many examples of such groups. A particularly interesting class of examples, derived from monodromy groups of iterated branch coverings, or equivalently from Galois groups of iterated polynomials, is presented. This class contains interesting groups from an algebraic point of view (just-non-solvable groups, groups of intermediate growth, branch groups,...), and at the same time the geometry of the group is apparent in that a limit of the group identifies naturally with the Julia set of the covering map. In its survey, the paper discusses finite-state transducers, growth of groups and languages, limit spaces of groups, hyperbolic spaces and groups, dynamical systems, Hecke-type operators, C^*-algebras, random matrices, ergodic theorems and entropy of non-commuting transformations. Self-similar groups appear then as a natural weaving thread through these seemingly different topics.
Automata, Groups, Limit Spaces, and Tilings
Laurent Bartholdi,Andre G. Henriques,Volodymyr V. Nekrashevych
Mathematics , 2004, DOI: 10.1016/j.jalgebra.2005.10.022
Abstract: We explore the connections between automata, groups, limit spaces of self-similar actions, and tilings. In particular, we show how a group acting ``nicely'' on a tree gives rise to a self-covering of a topological groupoid, and how the group can be reconstructed from the groupoid and its covering. The connection is via finite-state automata. These define decomposition rules, or self-similar tilings, on leaves of the solenoid associated with the covering.
Iterated Monodromy Groups
Volodymyr Nekrashevych
Mathematics , 2003,
Abstract: We associate a group $IMG(f)$ to every covering $f$ of a topological space $M$ by its open subset. It is the quotient of the fundamental group $\pi_1(M)$ by the intersection of the kernels of its monodromy action for the iterates $f^n$. Every iterated monodromy group comes together with a naturally defined action on a rooted tree. We present an effective method to compute this action and show how the dynamics of $f$ is related to the group. In particular, the Julia set of $f$ can be reconstructed from $\img(f)$ (from its action on the tree), if $f$ is expanding.
Hyperbolic groupoids and duality
Volodymyr Nekrashevych
Mathematics , 2011,
Abstract: We define hyperbolic groupoids, generalizing the notion of a Gromov hyperbolic group. Examples of hyperbolic groupoids include actions of Gromov hyperbolic groups on their boundaries, pseudogroups generated by expanding self-coverings, natural pseudogroups acting on leaves of stable (or unstable) foliation of an Anosov diffeomorphism, e.t.c.. We show that for every hyperbolic groupoid G there is a naturally defined dual groupoid G' acting on the Gromov boundary of a Cayley graph of G, which is also hyperbolic and such that (G')' is equivalent to G.
Smale spaces with virtually nilpotent splitting
Volodymyr Nekrashevych
Mathematics , 2012,
Abstract: We show that if (X, f) is a locally connected Smale space such that the local product structure on X can be lifted by a covering with virtually nilpotent group of deck transformations to a global direct product, then (X, f) is topologically conjugate to a hyperbolic infra-nilmanifold automorphism. We use this result to give a generalization to Smale spaces of a theorem of M. Brin and A. Manning on Anosov diffeomorphisms with pinched spectrum, and to show that every locally connected codimension one Smale space is topologically conjugate to a hyperbolic automorphism of a torus.
An uncountable family of 3-generated groups with isomorphic profinite completions
Volodymyr Nekrashevych
Mathematics , 2013,
Abstract: We construct an uncountable family of 3-generated residually finite just-infinite groups with isomorphic profinite completions. We also show that word growth rate is not a profinite property.
Finitely presented groups associated with expanding maps
Volodymyr Nekrashevych
Mathematics , 2013,
Abstract: We associate with every locally expanding self-covering $f:M\to M$ of a compact path connected metric space a finitely presented group $V_f$. We prove that this group is a complete invariant of the dynamical system: two groups $V_{f_1}$ and $V_{f_2}$ are isomorphic as abstract groups if and only if the corresponding dynamical systems are topologically conjugate. We also show that the commutator subgroup of $V_f$ is simple, and give a topological interpretation of $V_f/V_f'$.
Free subgroups in groups acting on rooted trees
Volodymyr Nekrashevych
Mathematics , 2008,
Abstract: We show that if a group $G$ acting faithfully on a rooted tree $T$ has a free subgroup, then either there exists a point $w$ of the boundary $\partial T$ and a free subgroup of $G$ with trivial stabilizer of $w$, or there exists $w\in\partial T$ and a free subgroup of $G$ fixing $w$ and acting faithfully on arbitrarily small neighborhoods of $w$. This can be used to prove absence of free subgroups for different known classes of groups. For instance, we prove that iterated monodromy groups of expanding coverings have no free subgroups and give another proof of a theorem by S. Sidki.
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