<p>For <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({\textbf {b}}=(b_1,\dots ,b_n)\in \mathbb {Z}_{&gt;0}^n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="bold">b</mi> <mo>=</mo> <mrow> <mo stretchy="false">(</mo> <msub> <mi>b</mi> <mn>1</mn> </msub> <mo>,</mo> <mo>⋯</mo> <mo>,</mo> <msub> <mi>b</mi> <mi>n</mi> </msub> <mo stretchy="false">)</mo> </mrow> <mo>∈</mo> <msubsup> <mi mathvariant="double-struck">Z</mi> <mrow> <mo>&gt;</mo> <mn>0</mn> </mrow> <mi>n</mi> </msubsup> </mrow> </math></EquationSource> </InlineEquation>, a <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\({\textbf {b}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="bold">b</mi> </math></EquationSource> </InlineEquation><i>-parking function</i> is defined to be a sequence <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\((\beta _1,\dots ,\beta _n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <msub> <mi>β</mi> <mn>1</mn> </msub> <mo>,</mo> <mo>⋯</mo> <mo>,</mo> <msub> <mi>β</mi> <mi>n</mi> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> of positive integers whose nondecreasing rearrangement <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\beta '_1\le \beta '_2\le \cdots \le \beta '_n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msubsup> <mi>β</mi> <mn>1</mn> <mo>′</mo> </msubsup> <mo>≤</mo> <msubsup> <mi>β</mi> <mn>2</mn> <mo>′</mo> </msubsup> <mo>≤</mo> <mo>⋯</mo> <mo>≤</mo> <msubsup> <mi>β</mi> <mi>n</mi> <mo>′</mo> </msubsup> </mrow> </math></EquationSource> </InlineEquation> satisfies <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\beta '_i\le b_1+\cdots + b_i\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msubsup> <mi>β</mi> <mi>i</mi> <mo>′</mo> </msubsup> <mo>≤</mo> <msub> <mi>b</mi> <mn>1</mn> </msub> <mo>+</mo> <mo>⋯</mo> <mo>+</mo> <msub> <mi>b</mi> <mi>i</mi> </msub> </mrow> </math></EquationSource> </InlineEquation>. The <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\({\textbf {b}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="bold">b</mi> </math></EquationSource> </InlineEquation>-parking-function polytope <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(\mathfrak {X}_{n}({\textbf {b}})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi mathvariant="fraktur">X</mi> <mi>n</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="bold">b</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is the convex hull of all <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\({\textbf {b}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="bold">b</mi> </math></EquationSource> </InlineEquation>-parking functions of length <i>n</i> in <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(\mathbb {R}^n\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mrow> <mi mathvariant="double-struck">R</mi> </mrow> <mi>n</mi> </msup> </math></EquationSource> </InlineEquation>. Geometric properties of <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(\mathfrak {X}_{n}({\textbf {b}})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi mathvariant="fraktur">X</mi> <mi>n</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="bold">b</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> were previously explored in the specific case where <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\({\textbf {b}}=(a,b,b,\dots ,b)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="bold">b</mi> <mo>=</mo> <mo stretchy="false">(</mo> <mi>a</mi> <mo>,</mo> <mi>b</mi> <mo>,</mo> <mi>b</mi> <mo>,</mo> <mo>⋯</mo> <mo>,</mo> <mi>b</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> and were shown to generalize those of the classical parking-function polytope. In this work, we study <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(\mathfrak {X}_{n}({\textbf {b}})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi mathvariant="fraktur">X</mi> <mi>n</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="bold">b</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> in full generality. We present a minimal inequality and vertex description for <InlineEquation ID="IEq13"> <EquationSource Format="TEX">\(\mathfrak {X}_{n}({\textbf {b}})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi mathvariant="fraktur">X</mi> <mi>n</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="bold">b</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>, prove it is a generalized permutahedron, and study its <i>h</i>-polynomial. Furthermore, we investigate <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\(\mathfrak {X}_{n}({\textbf {b}})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi mathvariant="fraktur">X</mi> <mi>n</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="bold">b</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> through the perspectives of building sets and polymatroids, allowing us to identify its combinatorial types and obtain bounds on its combinatorial and circuit diameters.</p>

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Combinatorics of Generalized Parking-Function Polytopes

  • Margaret M. Bayer,
  • Steffen Borgwardt,
  • Teressa Chambers,
  • Spencer Daugherty,
  • Aleyah Dawkins,
  • Danai Deligeorgaki,
  • Hsin-Chieh Liao,
  • Tyrrell McAllister,
  • Angela Morrison,
  • Garrett Nelson,
  • Andrés R. Vindas-Meléndez

摘要

For \({\textbf {b}}=(b_1,\dots ,b_n)\in \mathbb {Z}_{>0}^n\) b = ( b 1 , , b n ) Z > 0 n , a \({\textbf {b}}\) b -parking function is defined to be a sequence \((\beta _1,\dots ,\beta _n)\) ( β 1 , , β n ) of positive integers whose nondecreasing rearrangement \(\beta '_1\le \beta '_2\le \cdots \le \beta '_n\) β 1 β 2 β n satisfies \(\beta '_i\le b_1+\cdots + b_i\) β i b 1 + + b i . The \({\textbf {b}}\) b -parking-function polytope \(\mathfrak {X}_{n}({\textbf {b}})\) X n ( b ) is the convex hull of all \({\textbf {b}}\) b -parking functions of length n in \(\mathbb {R}^n\) R n . Geometric properties of \(\mathfrak {X}_{n}({\textbf {b}})\) X n ( b ) were previously explored in the specific case where \({\textbf {b}}=(a,b,b,\dots ,b)\) b = ( a , b , b , , b ) and were shown to generalize those of the classical parking-function polytope. In this work, we study \(\mathfrak {X}_{n}({\textbf {b}})\) X n ( b ) in full generality. We present a minimal inequality and vertex description for \(\mathfrak {X}_{n}({\textbf {b}})\) X n ( b ) , prove it is a generalized permutahedron, and study its h-polynomial. Furthermore, we investigate \(\mathfrak {X}_{n}({\textbf {b}})\) X n ( b ) through the perspectives of building sets and polymatroids, allowing us to identify its combinatorial types and obtain bounds on its combinatorial and circuit diameters.