Factors in infinite groups
Fuente:
arXiv
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| Autor principal: | |
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| Formato: | Preprint |
| Publicado: |
2026
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| _version_ | 1866914352972431360 |
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| author | Kabenyuk, Mikhail |
| author_facet | Kabenyuk, Mikhail |
| contents | Let $G$ be a group and $A\subseteq G$ a non-empty subset. A right $s$-factor associated with $A$ is a maximal subset $U\subseteq G$ such that the product $AU$ is direct. The lower and upper $s$-indices $|G:A|^-$ and $|G:A|^+$ are defined as the minimum and the supremum of the cardinalities of such maximal sets $U$. The subset $A$ is called stable if $|G:A|^- = |G:A|^+$, and $G$ is called stable if every subset of $G$ is stable.
Using a graph-theoretic reformulation in terms of Cayley graphs, we prove that every infinite group is unstable. Equivalently, for every infinite group $G$ there exists a subset $A\subseteq G$ for which maximal subsets $U$ with direct product $AU$ do not all have the same cardinality. This gives a negative answer to Question 21.58 of the Kourovka Notebook. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2602_22876 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Factors in infinite groups Kabenyuk, Mikhail Group Theory Combinatorics 20F99 (Primary) 05C25 (Secondary) Let $G$ be a group and $A\subseteq G$ a non-empty subset. A right $s$-factor associated with $A$ is a maximal subset $U\subseteq G$ such that the product $AU$ is direct. The lower and upper $s$-indices $|G:A|^-$ and $|G:A|^+$ are defined as the minimum and the supremum of the cardinalities of such maximal sets $U$. The subset $A$ is called stable if $|G:A|^- = |G:A|^+$, and $G$ is called stable if every subset of $G$ is stable. Using a graph-theoretic reformulation in terms of Cayley graphs, we prove that every infinite group is unstable. Equivalently, for every infinite group $G$ there exists a subset $A\subseteq G$ for which maximal subsets $U$ with direct product $AU$ do not all have the same cardinality. This gives a negative answer to Question 21.58 of the Kourovka Notebook. |
| title | Factors in infinite groups |
| topic | Group Theory Combinatorics 20F99 (Primary) 05C25 (Secondary) |
| url | https://arxiv.org/abs/2602.22876 |