<p>Let <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(R_n=K[x_1,\dots ,x_n]\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>R</mi> <mi>n</mi> </msub> <mo>=</mo> <mi>K</mi> <mrow> <mo stretchy="false">[</mo> <msub> <mi>x</mi> <mn>1</mn> </msub> <mo>,</mo> <mo>⋯</mo> <mo>,</mo> <msub> <mi>x</mi> <mi>n</mi> </msub> <mo stretchy="false">]</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> be the <i>n</i>-variable polynomial ring over a field <i>K</i>. Let <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(S_n\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>S</mi> <mi>n</mi> </msub> </math></EquationSource> </InlineEquation> denote the set of monomials in <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(R_n\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>R</mi> <mi>n</mi> </msub> </math></EquationSource> </InlineEquation>. A monomial <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(u \in S_n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>u</mi> <mo>∈</mo> <msub> <mi>S</mi> <mi>n</mi> </msub> </mrow> </math></EquationSource> </InlineEquation> is a <i>Gotzmann monomial</i> if its associated Borel-stable monomial ideal is a Gotzmann ideal. A longstanding open problem is to determine all Gotzmann monomials in <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(R_n\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>R</mi> <mi>n</mi> </msub> </math></EquationSource> </InlineEquation>. Given <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(u_0 \in S_{n-1}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>u</mi> <mn>0</mn> </msub> <mo>∈</mo> <msub> <mi>S</mi> <mrow> <mi>n</mi> <mo>-</mo> <mn>1</mn> </mrow> </msub> </mrow> </math></EquationSource> </InlineEquation>, its <i>Gotzmann threshold</i> is the unique non-negative integer <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(t_0=\tau _n(u_0)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>t</mi> <mn>0</mn> </msub> <mo>=</mo> <msub> <mi>τ</mi> <mi>n</mi> </msub> <mrow> <mo stretchy="false">(</mo> <msub> <mi>u</mi> <mn>0</mn> </msub> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> such that <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(u_0x_n^t\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>u</mi> <mn>0</mn> </msub> <msubsup> <mi>x</mi> <mi>n</mi> <mi>t</mi> </msubsup> </mrow> </math></EquationSource> </InlineEquation> is a Gotzmann monomial in <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(R_n\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>R</mi> <mi>n</mi> </msub> </math></EquationSource> </InlineEquation> if and only if <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(t \ge t_0\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>t</mi> <mo>≥</mo> <msub> <mi>t</mi> <mn>0</mn> </msub> </mrow> </math></EquationSource> </InlineEquation>. Currently, the function <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(\tau _n\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>τ</mi> <mi>n</mi> </msub> </math></EquationSource> </InlineEquation> is exactly known for <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(n \le 4\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≤</mo> <mn>4</mn> </mrow> </math></EquationSource> </InlineEquation> only. We present here an efficient procedure to determine <InlineEquation ID="IEq13"> <EquationSource Format="TEX">\(\tau _n(u_0)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>τ</mi> <mi>n</mi> </msub> <mrow> <mo stretchy="false">(</mo> <msub> <mi>u</mi> <mn>0</mn> </msub> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> for all <i>n</i> and all <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\(u_0 \in S_{n-1}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>u</mi> <mn>0</mn> </msub> <mo>∈</mo> <msub> <mi>S</mi> <mrow> <mi>n</mi> <mo>-</mo> <mn>1</mn> </mrow> </msub> </mrow> </math></EquationSource> </InlineEquation>. As an application, in the critical case <InlineEquation ID="IEq15"> <EquationSource Format="TEX">\(u_0=x_2^d\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>u</mi> <mn>0</mn> </msub> <mo>=</mo> <msubsup> <mi>x</mi> <mn>2</mn> <mi>d</mi> </msubsup> </mrow> </math></EquationSource> </InlineEquation>, we determine <InlineEquation ID="IEq16"> <EquationSource Format="TEX">\(\tau _5(x_2^d)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>τ</mi> <mn>5</mn> </msub> <mrow> <mo stretchy="false">(</mo> <msubsup> <mi>x</mi> <mn>2</mn> <mi>d</mi> </msubsup> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> for all <i>d</i> and we conjecture that for <InlineEquation ID="IEq17"> <EquationSource Format="TEX">\(n \ge 6\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≥</mo> <mn>6</mn> </mrow> </math></EquationSource> </InlineEquation>, <InlineEquation ID="IEq18"> <EquationSource Format="TEX">\(\tau _n(x_2^d)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>τ</mi> <mi>n</mi> </msub> <mrow> <mo stretchy="false">(</mo> <msubsup> <mi>x</mi> <mn>2</mn> <mi>d</mi> </msubsup> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is a polynomial in <i>d</i> of degree <InlineEquation ID="IEq19"> <EquationSource Format="TEX">\(2^{n-2}\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mn>2</mn> <mrow> <mi>n</mi> <mo>-</mo> <mn>2</mn> </mrow> </msup> </math></EquationSource> </InlineEquation> and dominant term equal to that of the <InlineEquation ID="IEq20"> <EquationSource Format="TEX">\((n-2)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <mi>n</mi> <mo>-</mo> <mn>2</mn> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>-iterated binomial coefficient <Equation ID="Equ22"> <EquationSource Format="TEX">\( \left( {\begin{array}{c}\left( {\begin{array}{c}\left( {\begin{array}{c}d\\ 2\end{array}}\right) \\ 2\end{array}}\right) \\ {\mathop {2}\limits ^{\cdots }}\end{array}}\right) . \)</EquationSource> <EquationSource Format="MATHML"><math display="block"> <mrow> <mfenced close=")" open="("> <mrow> <mtable> <mtr> <mtd> <mfenced close=")" open="("> <mrow> <mtable> <mtr> <mtd> <mfenced close=")" open="("> <mrow> <mtable> <mtr> <mtd> <mi>d</mi> </mtd> </mtr> <mtr> <mtd> <mrow> <mrow /> <mn>2</mn> </mrow> </mtd> </mtr> </mtable> </mrow> </mfenced> </mtd> </mtr> <mtr> <mtd> <mrow> <mrow /> <mn>2</mn> </mrow> </mtd> </mtr> </mtable> </mrow> </mfenced> </mtd> </mtr> <mtr> <mtd> <mrow> <mrow /> <mover> <mn>2</mn> <mo>⋯</mo> </mover> </mrow> </mtd> </mtr> </mtable> </mrow> </mfenced> <mo>.</mo> </mrow> </math></EquationSource> </Equation></p>

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On the Gotzmann threshold of monomials

  • Vittoria Bonanzinga,
  • Shalom Eliahou

摘要

Let \(R_n=K[x_1,\dots ,x_n]\) R n = K [ x 1 , , x n ] be the n-variable polynomial ring over a field K. Let \(S_n\) S n denote the set of monomials in \(R_n\) R n . A monomial \(u \in S_n\) u S n is a Gotzmann monomial if its associated Borel-stable monomial ideal is a Gotzmann ideal. A longstanding open problem is to determine all Gotzmann monomials in \(R_n\) R n . Given \(u_0 \in S_{n-1}\) u 0 S n - 1 , its Gotzmann threshold is the unique non-negative integer \(t_0=\tau _n(u_0)\) t 0 = τ n ( u 0 ) such that \(u_0x_n^t\) u 0 x n t is a Gotzmann monomial in \(R_n\) R n if and only if \(t \ge t_0\) t t 0 . Currently, the function \(\tau _n\) τ n is exactly known for \(n \le 4\) n 4 only. We present here an efficient procedure to determine \(\tau _n(u_0)\) τ n ( u 0 ) for all n and all \(u_0 \in S_{n-1}\) u 0 S n - 1 . As an application, in the critical case \(u_0=x_2^d\) u 0 = x 2 d , we determine \(\tau _5(x_2^d)\) τ 5 ( x 2 d ) for all d and we conjecture that for \(n \ge 6\) n 6 , \(\tau _n(x_2^d)\) τ n ( x 2 d ) is a polynomial in d of degree \(2^{n-2}\) 2 n - 2 and dominant term equal to that of the \((n-2)\) ( n - 2 ) -iterated binomial coefficient \( \left( {\begin{array}{c}\left( {\begin{array}{c}\left( {\begin{array}{c}d\\ 2\end{array}}\right) \\ 2\end{array}}\right) \\ {\mathop {2}\limits ^{\cdots }}\end{array}}\right) . \) d 2 2 2 .