<p>Partial permutohedra are lattice polytopes which were recently introduced and studied by Heuer and Striker. For positive integers <i>m</i> and <i>n</i>, the partial permutohedron&#xa0;<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({{\mathcal {P}}}(m,n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">P</mi> <mo stretchy="false">(</mo> <mi>m</mi> <mo>,</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is the convex hull of all vectors in <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\{0,1,\ldots ,n\}^m\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mrow> <mo stretchy="false">{</mo> <mn>0</mn> <mo>,</mo> <mn>1</mn> <mo>,</mo> <mo>…</mo> <mo>,</mo> <mi>n</mi> <mo stretchy="false">}</mo> </mrow> <mi>m</mi> </msup> </math></EquationSource> </InlineEquation> whose nonzero entries are distinct. We study the face lattice, volume and Ehrhart polynomial of <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\({{\mathcal {P}}}(m,n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">P</mi> <mo stretchy="false">(</mo> <mi>m</mi> <mo>,</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, and our methods and results include the following. For any <i>m</i> and <i>n</i>, we obtain a bijection between the nonempty faces of <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\({{\mathcal {P}}}(m,n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">P</mi> <mo stretchy="false">(</mo> <mi>m</mi> <mo>,</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> and certain chains of subsets of <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\{1,\dots ,m\}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">{</mo> <mn>1</mn> <mo>,</mo> <mo>⋯</mo> <mo>,</mo> <mi>m</mi> <mo stretchy="false">}</mo> </mrow> </math></EquationSource> </InlineEquation>, thereby confirming a conjecture of Heuer and Striker, and we then use this characterization of faces to obtain a closed expression for the <i>h</i>-polynomial of&#xa0;<InlineEquation ID="IEq6"> <EquationSource Format="TEX">\({{\mathcal {P}}}(m,n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">P</mi> <mo stretchy="false">(</mo> <mi>m</mi> <mo>,</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>. For any <i>m</i> and <i>n</i> with <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(n\ge m-1\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≥</mo> <mi>m</mi> <mo>-</mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation>, we use a pyramidal subdivision of <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\({\mathcal {P}}(m,n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">P</mi> <mo stretchy="false">(</mo> <mi>m</mi> <mo>,</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> to establish a recursive formula for the normalized volume of <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\({\mathcal {P}}(m,n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">P</mi> <mo stretchy="false">(</mo> <mi>m</mi> <mo>,</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, from which we then obtain closed expressions for this volume. We also use a sculpting process (in which <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\({{\mathcal {P}}}(m,n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">P</mi> <mo stretchy="false">(</mo> <mi>m</mi> <mo>,</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is reached by successively removing certain pieces from a simplex or hypercube) to obtain closed expressions for the Ehrhart polynomial of <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\({{\mathcal {P}}}(m,n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">P</mi> <mo stretchy="false">(</mo> <mi>m</mi> <mo>,</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> with arbitrary <i>m</i> and fixed <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(n\le 3\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≤</mo> <mn>3</mn> </mrow> </math></EquationSource> </InlineEquation>, the normalized volume of&#xa0;<InlineEquation ID="IEq13"> <EquationSource Format="TEX">\({{\mathcal {P}}}(m,4)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">P</mi> <mo stretchy="false">(</mo> <mi>m</mi> <mo>,</mo> <mn>4</mn> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> with arbitrary <i>m</i>, and the Ehrhart polynomial of <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\({{\mathcal {P}}}(m,n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">P</mi> <mo stretchy="false">(</mo> <mi>m</mi> <mo>,</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> with fixed <InlineEquation ID="IEq15"> <EquationSource Format="TEX">\(m\le 4\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>m</mi> <mo>≤</mo> <mn>4</mn> </mrow> </math></EquationSource> </InlineEquation> and arbitrary <InlineEquation ID="IEq16"> <EquationSource Format="TEX">\(n\ge m-1\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≥</mo> <mi>m</mi> <mo>-</mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation>.</p>

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Partial Permutohedra

  • Roger E. Behrend,
  • Federico Castillo,
  • Anastasia Chavez,
  • Alexander Diaz-Lopez,
  • Laura Escobar,
  • Pamela E. Harris,
  • Erik Insko

摘要

Partial permutohedra are lattice polytopes which were recently introduced and studied by Heuer and Striker. For positive integers m and n, the partial permutohedron  \({{\mathcal {P}}}(m,n)\) P ( m , n ) is the convex hull of all vectors in \(\{0,1,\ldots ,n\}^m\) { 0 , 1 , , n } m whose nonzero entries are distinct. We study the face lattice, volume and Ehrhart polynomial of \({{\mathcal {P}}}(m,n)\) P ( m , n ) , and our methods and results include the following. For any m and n, we obtain a bijection between the nonempty faces of \({{\mathcal {P}}}(m,n)\) P ( m , n ) and certain chains of subsets of \(\{1,\dots ,m\}\) { 1 , , m } , thereby confirming a conjecture of Heuer and Striker, and we then use this characterization of faces to obtain a closed expression for the h-polynomial of  \({{\mathcal {P}}}(m,n)\) P ( m , n ) . For any m and n with \(n\ge m-1\) n m - 1 , we use a pyramidal subdivision of \({\mathcal {P}}(m,n)\) P ( m , n ) to establish a recursive formula for the normalized volume of \({\mathcal {P}}(m,n)\) P ( m , n ) , from which we then obtain closed expressions for this volume. We also use a sculpting process (in which \({{\mathcal {P}}}(m,n)\) P ( m , n ) is reached by successively removing certain pieces from a simplex or hypercube) to obtain closed expressions for the Ehrhart polynomial of \({{\mathcal {P}}}(m,n)\) P ( m , n ) with arbitrary m and fixed \(n\le 3\) n 3 , the normalized volume of  \({{\mathcal {P}}}(m,4)\) P ( m , 4 ) with arbitrary m, and the Ehrhart polynomial of \({{\mathcal {P}}}(m,n)\) P ( m , n ) with fixed \(m\le 4\) m 4 and arbitrary \(n\ge m-1\) n m - 1 .