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D. Dikranjan

Publications and source records attributed to D. Dikranjan.

8 recordsLinked to original sources
arXiv2025

Nested ideals and topologically $\mathbf u_\mathcal I$-torsion elements of the circle group

Let $\mathbf u=(u_n)_{n\in\mathbb N}$ be a sequence in $\mathbb N_+$ with $u_0=1$ and $u_n\mid u_{n+1}$ for every $n\in\mathbb N$, and let $b_n:=u_{n+1}/u_n$ for every $n\in\mathbb N_+$. For every $r\in [0,1)$, there exists a unique sequence $(c_n)_{n\in\mathbb N_+}$ in $\mathbb N$ such that $r= \sum_{n=1}^\infty\frac{c_n}{u_n}$, with $c_n<b_n$ for every $n\in\mathbb N_+$, and $c_n<b_n-1$ for infinitely many $n\in\mathbb N_+$; let $\mathrm{supp}(r):=\{n\in\mathbb N_+: c_n\neq0\}$ and $\mathrm{supp}_b(r):=\{n\in\mathbb N_+: c_n = b_n-1\}$. For $x=r+\mathbb Z\in \mathbb T$, let $\mathrm{supp}(x) = \mathrm{supp}(r)$ and $\mathrm{supp}_b(x) = \mathrm{supp}_b(r)$. For an ideal $\mathcal I$ of $\mathbb N$, an element $x$ of the circle group $\mathbb T$ is called a topologically $\mathbf u_\mathcal I$-torsion element of $\mathbb T$ if $u_nx$ $\mathcal I$-converges to $0$, that is, $\{n\in \mathbb N: u_nx \not \in U\}\in \mathcal I$ for every neighborhood $U$ of $0$ in $\mathbb T$. In this paper, under suitable conditions on the ideal $\mathcal I$, we completely describe the $\mathbf u_\mathcal I$-torsion elements $x$ of $\mathbb T$ with $\lim_{n\in\mathrm{supp}(x)}b_n=\infty$ and those with $\{b_n:n\in\mathrm{supp}(x)\}$ bounded. According to Corollary 2.12 in [A. Ghosh, Ric. Mat. 73 (2024), 2263--2281], an element $x \in\mathbb T$ with $\{b_n:n\in\mathrm{supp}(x)\}$ bounded is topologically $\mathbf u_\mathcal I$-torsion if and only if $\mathrm{supp}(x)+1\setminus \mathrm{supp}(x)\in \mathcal I$ and $\mathrm{supp}(x) \setminus \mathrm{supp}_b(x) \in \mathcal I$. We characterize the ideals $\mathcal I$ of $\mathbb N$, naming them nested, such that this equivalence holds and we provide examples of non-nested ideals $\mathcal I$ that satisfy the above mentioned suitable conditions, so that the equivalence claimed by Ghosh fails for those $\mathcal I$.

math.GN↗
arXiv2019

Coarse structures on groups defined by $T$-sequences

A sequence $(a_{n}) $ in an Abelian group is called a $T$-sequence if there exists a Hausdorff group topology on $G$ in which $(a_{n}) $ converges to $0$. For a $T$-sequence $(a_{n}) $, $τ_{(a_{n}) } $ denotes the strongest group topology on $G$ in which $(a_{n}) $ converges to $0$. The ideal $\mathcal{I}_{(a_{n})} $ of all precompact subsets of $(G, τ_{(a_{n}) } )$ defines a coarse structure on $G$ with base of entourages $\{(x, y): x-y \in P \}$, $P\in\mathcal{I}_{(a_{n})}. $ We prove that $asdim \ \ (G, \mathcal{I}_{(a_{n}) }) =\infty $ for every non-trivial $T$-sequence $(a_{n})$ on $G$, and the coarse group $(G, \mathcal{I}_{(a_{n}) })$ has 1 end provided that $(a_{n}) $ generates $G$. The keypart play asymorphic copies of the Hamming space in $(G, \mathcal{I}_{(a_{n})})$.

math.GN↗
arXiv2019

Balleans, hyperballeans and ideals

A ballean $\mathcal{B}$ (or a coarse structure) on a set $X$ is a family of subsets of $X$ called balls (or entourages of the diagonal in $X\times X$) defined in such a way that $\mathcal{B}$ can be considered as the asymptotic counterpart of a uniform topological space. The aim of this paper is to study two concrete balleans defined by the ideals in the Boolean algebra of all subsets of $X$ and their hyperballeans, with particular emphasis on their connectedness structure, more specifically the number of their connected components.

math.GN↗
arXiv2019

Hyperballeans of groups

In this paper we define some ballean structure on the power set of a group and, in particular, we study the subballean with support the lattice of all its subgroups. If $G$ is a group, we denote by $L(G)$ the family of all subgroups of $G$. For two groups $G$ and $H$, we relate their algebraic structure via the ballean structure of $L(G)$ and $L(H)$.

math.GN↗
arXiv2013

Compact-like abelian groups without non-trivial quasi-convex null sequences

In this paper, we study precompact abelian groups G that contain no sequence {x_n} such that {0} \cup {\pm x_n : n \in N} is infinite and quasi-convex in G, and x_n --> 0. We characterize groups with this property in the following classes of groups: (a) bounded precompact abelian groups; (b) minimal abelian groups; (c) totally minimal abelian groups; (d) ω-bounded abelian groups. We also provide examples of minimal abelian groups with this property, and show that there exists a minimal pseudocompact abelian group with the same property; furthermore, under Martin's Axiom, the group may be chosen to be countably compact minimal abelian.

math.GN↗
arXiv2012

Characterizing sequences for precompact group topologies

A precompact group topology $τ$ on an abelian group $G$ is called {\em single sequence characterized} (for short, {\em ss-characterized}) if there is a sequence $\mathbf{u}= (u_n)$ in $G$ such that $τ$ is the finest precompact group topology on $G$ making $\mathbf{u}=(u_n)$ converge to zero. It is proved that a metrizable precompact abelian group $(G,τ)$ is $ss$-characterized iff it is countable. For every metrizable precompact group topology $τ$ on a countably infinite abelian group $G$ there exists a group topology $η$ such that $η$ is strictly finer than $τ$ and the groups $(G,τ)$ and $(G,η)$ have the equal Pontryagin dual groups. We give a complete description of all $ss$-characterized precompact abelian groups modulo countable $ss$-characterized groups from which we derive: (1) No infinite pseudocompact abelian group is $ss$-characterized. (2) An $ss$-characterized precompact abelian group is hereditarily disconnected.

math.GR↗
arXiv2012

On the quasi-component of pseudocompact abelian groups

In this paper, we describe the relationship between the quasi-component q(G) of a (perfectly) minimal pseudocompact abelian group G and the quasi-component q(\widetilde G) of its completion. Specifically, we characterize the pairs (C,A) of compact connected abelian groups C and subgroups A such that A \cong q(G) and C \cong q(\widetilde G). As a consequence, we show that for every positive integer n or n=ω, there exist plenty of abelian pseudocompact perfectly minimal n-dimensional groups G such that the quasi-component of G is not dense in the quasi-component of the completion of G.

math.GN↗