<p>By a coprime commutator in a profinite group <i>G</i> we mean any element of the form [<i>x</i>,&#xa0;<i>y</i>], where <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(x,y\in G\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>x</mi> <mo>,</mo> <mi>y</mi> <mo>∈</mo> <mi>G</mi> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\((|x|,|y|)=1\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <mo stretchy="false">|</mo> <mi>x</mi> <mo stretchy="false">|</mo> <mo>,</mo> <mo stretchy="false">|</mo> <mi>y</mi> <mo stretchy="false">|</mo> <mo stretchy="false">)</mo> <mo>=</mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation>. It is well-known that the subgroup generated by the coprime commutators of <i>G</i> is precisely the pronilpotent residual <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\gamma _\infty (G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>γ</mi> <mi>∞</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>. There are several recent works showing that finiteness conditions on the set of coprime commutators have strong impact on the properties of <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\gamma _\infty (G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>γ</mi> <mi>∞</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> and, more generally, on the structure of <i>G</i>. In this paper we show that if the set of coprime commutators of a profinite group <i>G</i> is covered by countably many procyclic subgroups, then <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\gamma _\infty (G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>γ</mi> <mi>∞</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is finite-by-procyclic. In particular, it follows that <i>G</i> is finite-by-pronilpotent-by-abelian.</p>

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Coprime commutators in profinite groups

  • Cristina Acciarri,
  • Pavel Shumyatsky

摘要

By a coprime commutator in a profinite group G we mean any element of the form [xy], where \(x,y\in G\) x , y G and \((|x|,|y|)=1\) ( | x | , | y | ) = 1 . It is well-known that the subgroup generated by the coprime commutators of G is precisely the pronilpotent residual \(\gamma _\infty (G)\) γ ( G ) . There are several recent works showing that finiteness conditions on the set of coprime commutators have strong impact on the properties of \(\gamma _\infty (G)\) γ ( G ) and, more generally, on the structure of G. In this paper we show that if the set of coprime commutators of a profinite group G is covered by countably many procyclic subgroups, then \(\gamma _\infty (G)\) γ ( G ) is finite-by-procyclic. In particular, it follows that G is finite-by-pronilpotent-by-abelian.