<p>Freiman conjectured that if <i>S</i> is a finite subset of a torsion-free group <i>G</i> with <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(k\ge 3\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>k</mi> <mo>≥</mo> <mn>3</mn> </mrow> </math></EquationSource> </InlineEquation> elements and <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(|S^{2}|\le 3k-4,\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mrow> <mo stretchy="false">|</mo> </mrow> <msup> <mi>S</mi> <mn>2</mn> </msup> <mrow> <mo stretchy="false">|</mo> <mo>≤</mo> <mn>3</mn> <mi>k</mi> <mo>-</mo> <mn>4</mn> <mo>,</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> then <i>S</i> is a subset of a small geometric progression of length at most <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(2k-3\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mn>2</mn> <mi>k</mi> <mo>-</mo> <mn>3</mn> </mrow> </math></EquationSource> </InlineEquation>. In 2014, Freiman et al. settled this conjecture when <i>S</i> is a finite subset of an ordered group. In this article, we focus on right-ordered groups and prove Freiman’s conjecture when <i>S</i> is a finite subset of a right-ordered group under certain restrictions on the set <i>S</i>. Further, we consider the right-ordered Baumslag-Solitar group <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\textrm{BS}(1,q) =\langle a,b: ab = b^qa\rangle \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mtext>BS</mtext> <mrow> <mo stretchy="false">(</mo> <mn>1</mn> <mo>,</mo> <mi>q</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <mrow> <mo stretchy="false">⟨</mo> <mi>a</mi> <mo>,</mo> <mi>b</mi> <mo>:</mo> <mi>a</mi> <mi>b</mi> <mo>=</mo> <msup> <mi>b</mi> <mi>q</mi> </msup> <mi>a</mi> <mo stretchy="false">⟩</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>, where <i>q</i> is an integer. We answer Freiman’s conjecture for <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\textrm{BS}(1,q)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mtext>BS</mtext> <mo stretchy="false">(</mo> <mn>1</mn> <mo>,</mo> <mi>q</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(q\ne -1\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>q</mi> <mo>≠</mo> <mo>-</mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation> by proving that if <i>S</i> is a finite subset of <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(\textrm{BS}(1,q)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mtext>BS</mtext> <mo stretchy="false">(</mo> <mn>1</mn> <mo>,</mo> <mi>q</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(q\ne -1\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>q</mi> <mo>≠</mo> <mo>-</mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation> with the identity element as its minimum and <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(|S^2|\le 3|S|-4\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mrow> <mo stretchy="false">|</mo> </mrow> <msup> <mi>S</mi> <mn>2</mn> </msup> <mrow> <mo stretchy="false">|</mo> <mo>≤</mo> <mn>3</mn> <mo stretchy="false">|</mo> <mi>S</mi> <mo stretchy="false">|</mo> <mo>-</mo> <mn>4</mn> </mrow> </mrow> </math></EquationSource> </InlineEquation>, then the subgroup generated by the set <i>S</i> is an abelian subgroup of <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(\textrm{BS}(1,q)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mtext>BS</mtext> <mo stretchy="false">(</mo> <mn>1</mn> <mo>,</mo> <mi>q</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>.</p>

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On Small Doubling in Right-Ordered Groups and Baumslag-Solitar Groups

  • Mohan,
  • Neetu,
  • B R Shankar

摘要

Freiman conjectured that if S is a finite subset of a torsion-free group G with \(k\ge 3\) k 3 elements and \(|S^{2}|\le 3k-4,\) | S 2 | 3 k - 4 , then S is a subset of a small geometric progression of length at most \(2k-3\) 2 k - 3 . In 2014, Freiman et al. settled this conjecture when S is a finite subset of an ordered group. In this article, we focus on right-ordered groups and prove Freiman’s conjecture when S is a finite subset of a right-ordered group under certain restrictions on the set S. Further, we consider the right-ordered Baumslag-Solitar group \(\textrm{BS}(1,q) =\langle a,b: ab = b^qa\rangle \) BS ( 1 , q ) = a , b : a b = b q a , where q is an integer. We answer Freiman’s conjecture for \(\textrm{BS}(1,q)\) BS ( 1 , q ) , \(q\ne -1\) q - 1 by proving that if S is a finite subset of \(\textrm{BS}(1,q)\) BS ( 1 , q ) , \(q\ne -1\) q - 1 with the identity element as its minimum and \(|S^2|\le 3|S|-4\) | S 2 | 3 | S | - 4 , then the subgroup generated by the set S is an abelian subgroup of \(\textrm{BS}(1,q)\) BS ( 1 , q ) .