<p>For a finitely generated LEF group <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\Gamma \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">Γ</mi> </math></EquationSource> </InlineEquation>, we study the orders of finite groups admitting local embeddings of balls in a word metric on <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\Gamma \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">Γ</mi> </math></EquationSource> </InlineEquation>, as measured by the <i>LEF growth function</i>. We prove that any sufficiently smooth increasing function between <i>n</i>! and <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\exp (\exp (n))\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>exp</mo> <mo stretchy="false">(</mo> <mo>exp</mo> <mo stretchy="false">(</mo> <mi>n</mi> <mo stretchy="false">)</mo> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is close to the LEF growth function of some finitely generated group. This is achieved by estimating the LEF growth of some semidirect products of the form <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\({{\,\textrm{FSym}\,}}(\Omega ) \rtimes \Gamma \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mrow> <mspace width="0.166667em" /> <mtext>FSym</mtext> <mspace width="0.166667em" /> </mrow> <mo stretchy="false">(</mo> <mi mathvariant="normal">Ω</mi> <mo stretchy="false">)</mo> <mo>⋊</mo> <mi mathvariant="normal">Γ</mi> </mrow> </math></EquationSource> </InlineEquation>, where <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\Omega \curvearrowleft \Gamma \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="normal">Ω</mi> <mo>↶</mo> <mi mathvariant="normal">Γ</mi> </mrow> </math></EquationSource> </InlineEquation> is an appropriate transitive action and <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\({{\,\textrm{FSym}\,}}(\Omega )\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mrow> <mspace width="0.166667em" /> <mtext>FSym</mtext> <mspace width="0.166667em" /> </mrow> <mo stretchy="false">(</mo> <mi mathvariant="normal">Ω</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is the group of finitely supported permutations of <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(\Omega \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">Ω</mi> </math></EquationSource> </InlineEquation>. A key tool in the proof is to identify sequences of finitely presented subgroups with short “relative” presentations. In a similar vein, we also obtain estimates on the LEF growth of some groups of the form <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(E_{\Omega } (R) \rtimes \Gamma \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>E</mi> <mi mathvariant="normal">Ω</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>R</mi> <mo stretchy="false">)</mo> </mrow> <mo>⋊</mo> <mi mathvariant="normal">Γ</mi> </mrow> </math></EquationSource> </InlineEquation>, for <i>R</i> an appropriate unital ring and <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(E_{\Omega } (R)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>E</mi> <mi mathvariant="normal">Ω</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>R</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> the subgroup of <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\({{\,\textrm{Aut}\,}}_R (R[\Omega ])\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mrow> <mspace width="0.166667em" /> <mtext>Aut</mtext> <mspace width="0.166667em" /> </mrow> <mi>R</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>R</mi> <mrow> <mo stretchy="false">[</mo> <mi mathvariant="normal">Ω</mi> <mo stretchy="false">]</mo> </mrow> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> generated by all transvections with respect to basis <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(\Omega \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">Ω</mi> </math></EquationSource> </InlineEquation>.</p>

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Controlling LEF growth in some group extensions

  • Henry Bradford

摘要

For a finitely generated LEF group \(\Gamma \) Γ , we study the orders of finite groups admitting local embeddings of balls in a word metric on \(\Gamma \) Γ , as measured by the LEF growth function. We prove that any sufficiently smooth increasing function between n! and \(\exp (\exp (n))\) exp ( exp ( n ) ) is close to the LEF growth function of some finitely generated group. This is achieved by estimating the LEF growth of some semidirect products of the form \({{\,\textrm{FSym}\,}}(\Omega ) \rtimes \Gamma \) FSym ( Ω ) Γ , where \(\Omega \curvearrowleft \Gamma \) Ω Γ is an appropriate transitive action and \({{\,\textrm{FSym}\,}}(\Omega )\) FSym ( Ω ) is the group of finitely supported permutations of \(\Omega \) Ω . A key tool in the proof is to identify sequences of finitely presented subgroups with short “relative” presentations. In a similar vein, we also obtain estimates on the LEF growth of some groups of the form \(E_{\Omega } (R) \rtimes \Gamma \) E Ω ( R ) Γ , for R an appropriate unital ring and \(E_{\Omega } (R)\) E Ω ( R ) the subgroup of \({{\,\textrm{Aut}\,}}_R (R[\Omega ])\) Aut R ( R [ Ω ] ) generated by all transvections with respect to basis \(\Omega \) Ω .