<p>According to a well-known result in geometric topology, we have <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\left( \mathbb {S}^2 \right) ^{n}\!\!/\operatorname {Sym}(n) = \mathbb{C}\mathbb{P}^{n}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msup> <mfenced close=")" open="("> <msup> <mrow> <mi mathvariant="double-struck">S</mi> </mrow> <mn>2</mn> </msup> </mfenced> <mi>n</mi> </msup> <mspace width="-0.166667em" /> <mspace width="-0.166667em" /> <mo stretchy="false">/</mo> <mo>Sym</mo> <mrow> <mo stretchy="false">(</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <mi mathvariant="double-struck">C</mi> <msup> <mrow> <mi mathvariant="double-struck">P</mi> </mrow> <mi>n</mi> </msup> </mrow> </math></EquationSource> </InlineEquation>, where <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\operatorname {Sym}(n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>Sym</mo> <mo stretchy="false">(</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> acts on <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\left( \mathbb {S}^2 \right) ^{n}\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mfenced close=")" open="("> <msup> <mrow> <mi mathvariant="double-struck">S</mi> </mrow> <mn>2</mn> </msup> </mfenced> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation> by coordinate permutation. We use this fact to explicitly construct a regular simplicial cell decomposition of <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(\mathbb{C}\mathbb{P}^{n}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="double-struck">C</mi> <msup> <mrow> <mi mathvariant="double-struck">P</mi> </mrow> <mi>n</mi> </msup> </mrow> </math></EquationSource> </InlineEquation> for each <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(n \ge 2\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≥</mo> <mn>2</mn> </mrow> </math></EquationSource> </InlineEquation>. In more detail, we start with the standard two triangle crystallisation <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(S^2_3\)</EquationSource> <EquationSource Format="MATHML"><math> <msubsup> <mi>S</mi> <mn>3</mn> <mn>2</mn> </msubsup> </math></EquationSource> </InlineEquation> of the 2-sphere <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(\mathbb {S}^2\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mrow> <mi mathvariant="double-struck">S</mi> </mrow> <mn>2</mn> </msup> </math></EquationSource> </InlineEquation>, in its <i>n</i>-fold Cartesian product. We then construct a simplicial subdivision of this product and prove that the <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(\operatorname {Sym}(n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>Sym</mo> <mo stretchy="false">(</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> quotient of this subdivision yields a simplicial cell decomposition of <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(\mathbb{C}\mathbb{P}^n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="double-struck">C</mi> <msup> <mrow> <mi mathvariant="double-struck">P</mi> </mrow> <mi>n</mi> </msup> </mrow> </math></EquationSource> </InlineEquation>. The first derived subdivision of this cell complex is a simplicial triangulation of <InlineEquation ID="IEq13"> <EquationSource Format="TEX">\(\mathbb{C}\mathbb{P}^n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="double-struck">C</mi> <msup> <mrow> <mi mathvariant="double-struck">P</mi> </mrow> <mi>n</mi> </msup> </mrow> </math></EquationSource> </InlineEquation>. To the best of our knowledge, this is the first explicit description of triangulations of <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\(\mathbb{C}\mathbb{P}^n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="double-struck">C</mi> <msup> <mrow> <mi mathvariant="double-struck">P</mi> </mrow> <mi>n</mi> </msup> </mrow> </math></EquationSource> </InlineEquation> for <InlineEquation ID="IEq15"> <EquationSource Format="TEX">\(n \ge 4.\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≥</mo> <mn>4</mn> <mo>.</mo> </mrow> </math></EquationSource> </InlineEquation></p>

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Simplicial cell decompositions of \(\mathbb{C}\mathbb{P}^{\hspace{.3mm}n}\)

  • Basudeb Datta,
  • Jonathan Spreer

摘要

According to a well-known result in geometric topology, we have \(\left( \mathbb {S}^2 \right) ^{n}\!\!/\operatorname {Sym}(n) = \mathbb{C}\mathbb{P}^{n}\) S 2 n / Sym ( n ) = C P n , where \(\operatorname {Sym}(n)\) Sym ( n ) acts on \(\left( \mathbb {S}^2 \right) ^{n}\) S 2 n by coordinate permutation. We use this fact to explicitly construct a regular simplicial cell decomposition of \(\mathbb{C}\mathbb{P}^{n}\) C P n for each \(n \ge 2\) n 2 . In more detail, we start with the standard two triangle crystallisation \(S^2_3\) S 3 2 of the 2-sphere \(\mathbb {S}^2\) S 2 , in its n-fold Cartesian product. We then construct a simplicial subdivision of this product and prove that the \(\operatorname {Sym}(n)\) Sym ( n ) quotient of this subdivision yields a simplicial cell decomposition of \(\mathbb{C}\mathbb{P}^n\) C P n . The first derived subdivision of this cell complex is a simplicial triangulation of \(\mathbb{C}\mathbb{P}^n\) C P n . To the best of our knowledge, this is the first explicit description of triangulations of \(\mathbb{C}\mathbb{P}^n\) C P n for \(n \ge 4.\) n 4 .