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Artem Dudko

Publications and source records attributed to Artem Dudko.

29 records · Page 2Linked to original sources
arXiv2016

On spectra of Koopman, groupoid and quasi-regular representations

In this paper we investigate relations between Koopman, groupoid and quasi-regular representations of countable groups. We show that for an ergodic measure class preserving action of a countable group G on a standard Borel space the associated groupoid and quasi-regular representations are weakly equivalent and weakly contained in the Koopman representation. Moreover, if the action is hyperfinite then the Koopman representation is weakly equivalent to the groupoid. As a corollary of our results we obtain a continuum of pairwise disjoint pairwise equivalent irreducible representations of weakly branch groups. As an illustration we calculate spectra of regular, Koopman and groupoid representations associated to the action of the 2-group of intermediate growth constructed by the second author in 1980.

math.RT↗
arXiv2016

On irreducibility and disjointness of Koopman and quasi-regular representations of weakly branch groups

We study Koopman and quasi-regular representations corresponding to the action of arbitrary weakly branch group G on the boundary of a rooted tree T. One of the main results is that in the case of a quasi-invariant Bernoulli measure on the boundary of T the corresponding Koopman representation of G is irreducible (under some general conditions). We also show that quasi-regular representations of G corresponding to different orbits and Koopman representations corresponding to different Bernoulli measures on the boundary of T are pairwise disjoint. This gives two continual collections of pairwise disjoint irreducible representations of a weakly branch group. Another corollary of our results is triviality of the centralizer of G in various groups of transformations on the boundary of T.

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arXiv2016

On irreducibility of Koopman representations of Higman-Thompson groups

We introduce a notion of measure contracting actions and show that Koopman representations corresponding to ergodic measure contracting actions are irreducible. As a corollary we obtain that Koopman representations associated to canonical actions of Higman-Thompson groups are irreducible. We also show that the actions of weakly branch groups on the boundaries of rooted trees are measure contracting. This gives a new point of view on irreducibility of the corresponding Koopman representations.

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arXiv2014

On the resurgent approach to Ecalle-Voronin's invariants

Given a holomorphic germ at the origin of C with a simple parabolic fixed point, the Fatou coordinates have a common asymptotic expansion whose formal Borel transform is resurgent. We show how to use Ecalle's alien operators to study the singularities in the Borel plane and relate them to the horn maps, providing each of Ecalle-Voronin's invariants in the form of a convergent numerical series. The proofs are original and self-contained, with ordinary Borel summability as the only prerequisite.

math.DS↗
arXiv2014

The resurgent character of the Fatou coordinates of a simple parabolic germ

Given a holomorphic germ at the origin of C with a simple parabolic fixed point, the local dynamics is classically described by means of pairs of attracting and repelling Fatou coordinates and the corresponding pairs of horn maps, of crucial importance for Écalle-Voronin's classification result and the definition of the parabolic renormalization operator. We revisit Écalle's approach to the construction of Fatou coordinates, which relies on Borel-Laplace summation, and give an original and self-contained proof of their resurgent character.

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arXiv2012

Finite Factor Representations of Higman-Thompson groups

We prove that the only finite factor-representations of the Higman-Thompson groups $\{F_{n,r}\}$, $ \{G_{n,r}\}$ are the regular representations and scalar representations arising from group abelianizations. As a corollary, we obtain that any measure-preserving ergodic action of a simple Higman-Thompson group must be essentially free. Finite factor representations of other classes of groups are also discussed.

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arXiv2011

On Characters of Inductive Limits of Symmetric Groups

In the paper we completely describe characters (central positive-definite functions) of simple locally finite groups that can be represented as inductive limits of (products of) symmetric groups under block diagonal embeddings. Each such group $G$ defines an infinite graph (Bratteli diagram) that encodes the embedding scheme. The group $G$ acts on the space $X$ of infinite paths of the associated Bratteli diagram by changing initial edges of paths. Assuming the finiteness of the set of ergodic measures for the system $(X,G)$, we establish that each indecomposable character $χ:G \rightarrow \mathbb C$ is uniquely defined by the formula $χ(g) = μ_1(Fix(g))^{α_1}...μ_k(Fix(g))^{α_k}$, where $μ_1,...,μ_k$ are $G$-ergodic measures, $Fix(g) = \{x\in X: gx = x\}$, and $α_1,...,α_k\in \{0,1,...,\infty\}$. We illustrate our results on the group of rational permutations of the unit interval.

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arXiv2010

Characters on the Full Group of the Odometer

Let $X$ be the space of all infinite $0,1$-sequences and $T$ be the odometer on $X$. In this paper we introduce a dense subgroup $S(2^\infty)$ of the full group $[T]$ and describe all indecomposable characters on $S(2^\infty)$. As result we obtain a description of indecomposable characters on $[T]$.

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