<p>In this paper, we show that the Picard modular group <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="12044_2025_845_Article_IEq3.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="91" /> </InlineMediaObject> <EquationSource Format="TEX">\({PU}(2, 1; \mathcal {O}_7)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mrow> <mi mathvariant="italic">PU</mi> </mrow> <mo stretchy="false">(</mo> <mn>2</mn> <mo>,</mo> <mn>1</mn> <mo>;</mo> <msub> <mi mathvariant="script">O</mi> <mn>7</mn> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> can be generated by five explicitly given transformations. First using the Langlands decomposition we will see that three Heisenberg translations together with a Heisenberg rotation generate the &#xa0;stabilizer subgroup of infinity. Then through a geometric argument and also an algebraic &#xa0;argument we show that adjoining an inversion to the transformations described previously generates <InlineEquation ID="IEq4"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="12044_2025_845_Article_IEq4.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="96" /> </InlineMediaObject> <EquationSource Format="TEX">\(PU(2, 1; \mathcal {O}_7).\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>P</mi> <mi>U</mi> <mo stretchy="false">(</mo> <mn>2</mn> <mo>,</mo> <mn>1</mn> <mo>;</mo> <msub> <mi mathvariant="script">O</mi> <mn>7</mn> </msub> <mo stretchy="false">)</mo> <mo>.</mo> </mrow> </math></EquationSource> </InlineEquation> The idea behind the geometric method is finding a union of isometric spheres such that a fundamental domain for the stabilizer subgroup of infinity lies inside their boundaries. And the idea behind the algebraic method is to implement the continued fraction algorithm.</p>

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A geometric and an algebraic approach to find generators for the Picard modular group \(\varvec{PU(2, 1; \mathcal {O}_7)}\)

  • Majid Heydarpour,
  • Ehsan Rasoulian

摘要

In this paper, we show that the Picard modular group \({PU}(2, 1; \mathcal {O}_7)\) PU ( 2 , 1 ; O 7 ) can be generated by five explicitly given transformations. First using the Langlands decomposition we will see that three Heisenberg translations together with a Heisenberg rotation generate the  stabilizer subgroup of infinity. Then through a geometric argument and also an algebraic  argument we show that adjoining an inversion to the transformations described previously generates \(PU(2, 1; \mathcal {O}_7).\) P U ( 2 , 1 ; O 7 ) . The idea behind the geometric method is finding a union of isometric spheres such that a fundamental domain for the stabilizer subgroup of infinity lies inside their boundaries. And the idea behind the algebraic method is to implement the continued fraction algorithm.