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JOURNALS // Journal of Siberian Federal University. Mathematics & Physics // Archive

J. Sib. Fed. Univ. Math. Phys., 2024 Volume 17, Issue 5, Pages 684–688 (Mi jsfu1200)

To the question of the closure of the carpet

Elizaveta N. Troyanskaya

Siberian Federal University, Krasnoyarsk, Russian Federation

Abstract: For a root system $\Phi$, the set $\mathfrak{A}= \{\mathfrak{A}_{r}\ | \ r \in \Phi \}$ of additive subgroups $\mathfrak{A}_{r}$ over commutative ring $K$ is called a carpet of type $\Phi$ if commuting two root elements $x_{r}(t), t \in \mathfrak{A}_{r}$ and $x_{s}(u), u \in \mathfrak{A}_{s}$, gives a result where each factor lies in the subgroup $\Phi (\mathfrak{A})$ generated by the root elements $x_{r}(t), t \in \mathfrak{A}_{r}, r \in \Phi$. The subgroup $\Phi (\mathfrak{A})$ is called a carpet subgroup. It defines a new set of additive subgroups $\overline{\mathfrak{A}} = \{\overline{\mathfrak{A}}_{r} | r \in \Phi \}$, the name of the closure of the carpet $\mathfrak{A}$, which is set by equation $\overline{\mathfrak{A}}_{r} = \{t \in K\ | \ x_{r}(t) \in \Phi(\mathfrak{A})\}$. Ya. Nuzhin wrote down the following question in the Kourovka notebook. Is the closure $\overline{\mathfrak{A}}$ of a carpet $\mathfrak{A}$ {\it a carpet too? (question 19.61). The article provides a partial answer to this question. It is proved that the closure of a carpet of type $\Phi$ over commutative ring of odd characteristic $p$ is a carpet if $3$ does not divide $p$ when $\Phi$ of type $G_{2}$.

Keywords: commutative ring, Chevalley group, carpet of additive subgroups, $K$-character.

UDC: 512.54

Received: 10.03.2024
Received in revised form: 15.04.2024
Accepted: 17.05.2024

Language: English



© Steklov Math. Inst. of RAS, 2025