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Commutators of relative and unrelative elementary subgroups in Chevalley groups

In the present paper, which is a direct sequel of our papers [10,11,35] joint with Roozbeh Hazrat, we achieve a further dramatic reduction of the generating sets for commutators of relative elementary subgroups in Chevalley groups. Namely, let $Φ$ be a reduced irreducible root system of rank $\ge 2$, let $R$ be a commutative ring and let $A,B$ be two ideals of $R$. We consider subgroups of the Chevalley group $G(Φ,R)$ of type $Φ$ over $R$. The unrelative elementary subgroup $E(Φ,A)$ of level $A$ is generated (as a group) by the elementary unipotents $x_α(a)$, $α\inΦ$, $a\in A$, of level $A$. Its normal closure in the absolute elementary subgroup $E(Φ,R)$ is denoted by $E(Φ,R,A)$ and is called the relative elementary subgroup of level $A$. The main results of [11,35] consisted in construction of economic generator sets for the mutual commutator subgroups $[E(Φ,R,A),E(Φ,R,B)]$, where $A$ and $B$ are two ideals of $R$. It turned out that one can take Stein---Tits---Vaserstein generators of $E(Φ,R,AB)$, plus elementary commutators of the form $y_α(a,b)=[x_α(a),x_{-α}(b)]$, where $a\in A$, $b\in B$. Here we improve these results even further, by showing that in fact it suffices to engage only elementary commutators corresponding to {\it one\/} long root, and that modulo $E(Φ,R,AB)$ the commutators $y_α(a,b)$ behave as symbols. We discuss also some further variations and applications of these results.