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Register a new userFree planar actions of the Klein bottle group
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Authors
Frederic Le Roux
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We describe the structure of the free actions of the Klein bottle group by orientation preserving homeomorphisms of the plane. This group is generated by two elements $a,b$, where the conjugate of $b$ by $a$ equals the inverse of $b$. The main result is that $a$ must act properly discontinuously, while $b$ cannot act properly discontinuously. As a corollary, we describe some torsion free groups that cannot act freely on the plane. We also find some properties which are reminiscent of Brouwer theory for the infinite cyclic group $Z$, in particular that every free action is virtually wandering.
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A Cantor action is a minimal equicontinuous action of a countably generated group G on a Cantor space X. Such actions are also called generalized odometers in the literature. In this work, we introduce two new conjugacy invariants for Cantor actions, the stabilizer limit group and the centralizer limit group. An action is wild if the stabilizer limit group is an increasing sequence of stabilizer groups without bound, and otherwise is said to be stable if this group chain is bounded. For Cantor actions by a finitely generated group G, we prove that stable actions satisfy a rigidity principle, and furthermore show that the wild property is an invariant of the continuous orbit equivalence class of the action. A Cantor action is said to be dynamically wild if it is wild, and the centralizer limit group is a proper subgroup of the stabilizer limit group. This property is also a conjugacy invariant, and we show that a Cantor action with a non-Hausdorff element must be dynamically wild. We then give examples of wild Cantor actions with non-Hausdorff elements, using recursive methods from Geometric Group Theory to define actions on the boundaries of trees.
Let $H_3(Bbb R)$ denote the 3-dimensional real Heisenberg group. Given a family of lattices $Gamma_1supsetGamma_2supsetcdots$ in it, let $T$ stand for the associated uniquely ergodic $H_3(Bbb R)$-{it odometer}, i.e. the inverse limit of the $H_3(Bbb R)$-actions by rotations on the homogeneous spaces $H_3(Bbb R)/Gamma_j$, $jinBbb N$. The decomposition of the underlying Koopman unitary representation of $H_3(Bbb R)$ into a countable direct sum of irreducible components is explicitly described. The ergodic 2-fold self-joinings of $T$ are found. It is shown that in general, the $H_3(Bbb R)$-odometers are neither isospectral nor spectrally determined.
In this paper we study chaotic behavior of actions of a countable discrete group acting on a compact metric space by self-homeomorphisms. For actions of a countable discrete group G, we introduce local weak mixing and Li-Yorke chaos; and prove that local weak mixing implies Li-Yorke chaos if G is infinite, and positive topological entropy implies local weak mixing if G is an infinite countable discrete amenable group. Moreover, when considering a shift of finite type for actions of an infinite countable amenable group G, if the action has positive topological entropy then its homoclinic equivalence relation is non-trivial, and the converse holds true if additionally G is residually finite and the action contains a dense set of periodic points.
In this work, we investigate the dynamical and geometric properties of weak solenoids, as part of the development of a calculus of group chains associated to Cantor minimal actions. The study of the properties of group chains was initiated in the works of McCord 1965 and Fokkink and Oversteegen 2002, to study the problem of determining which weak solenoids are homogeneous continua. We develop an alternative condition for the homogeneity in terms of the Ellis semigroup of the action, then investigate the relationship between non-homogeneity of a weak solenoid and its discriminant invariant, which we introduce in this work. A key part of our study is the construction of new examples that illustrate various subtle properties of group chains that correspond to geometric properties of non-homogeneous weak solenoids.
An textit{algebraic} action of a discrete group $Gamma $ is a homomorphism from $Gamma $ to the group of continuous automorphisms of a compact abelian group $X$. By duality, such an action of $Gamma $ is determined by a module $M=widehat{X}$ over the integer group ring $mathbb{Z}Gamma $ of $Gamma $. The simplest examples of such modules are of the form $M=mathbb{Z}Gamma /mathbb{Z}Gamma f$ with $fin mathbb{Z}Gamma $; the corresponding algebraic action is the textit{principal algebraic $Gamma $-action} $alpha _f$ defined by $f$. In this note we prove the following extensions of results by Hayes cite{Hayes} on ergodicity of principal algebraic actions: If $Gamma $ is a countably infinite discrete group which is not virtually cyclic, and if $finmathbb{Z}Gamma $ satisfies that right multiplication by $f$ on $ell ^2(Gamma ,mathbb{R})$ is injective, then the principal $Gamma $-action $alpha _f$ is ergodic (Theorem ref{t:ergodic2}). If $Gamma $ contains a finitely generated subgroup with a single end (e.g. a finitely generated amenable subgroup which is not virtually cyclic), or an infinite nonamenable subgroup with vanishing first $ell ^2$-Betti number (e.g., an infinite property $T$ subgroup), the injectivity condition on $f$ can be replaced by the weaker hypothesis that $f$ is not a right zero-divisor in $mathbb{Z}Gamma $ (Theorem ref{t:ergodic1}). Finally, if $Gamma $ is torsion-free, not virtually cyclic, and satisfies Linnells textit{analytic zero-divisor conjecture}, then $alpha _f$ is ergodic for every $fin mathbb{Z}Gamma $ (Remark ref{r:analytic zero divisor}).
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