<p>A monomial ideal <i>I</i> is said to have homological linear quotients if, for each <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(k\ge 0\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>k</mi> <mo>≥</mo> <mn>0</mn> </mrow> </math></EquationSource> </InlineEquation>, the homological shift ideal <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\({{\,\textrm{HS}\,}}_k(I)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mrow> <mspace width="0.166667em" /> <mtext>HS</mtext> <mspace width="0.166667em" /> </mrow> <mi>k</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>I</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> has linear quotients. It is a well-known fact that if an edge ideal <i>I</i>(<i>G</i>) has homological linear quotients, then <i>G</i> is co-chordal. We construct a family of co-chordal graphs <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\{{{\,\textrm{H}\,}}_n^c\}_{n\ge 6}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mrow> <mo stretchy="false">{</mo> <msubsup> <mrow> <mspace width="0.166667em" /> <mtext>H</mtext> <mspace width="0.166667em" /> </mrow> <mi>n</mi> <mi>c</mi> </msubsup> <mo stretchy="false">}</mo> </mrow> <mrow> <mi>n</mi> <mo>≥</mo> <mn>6</mn> </mrow> </msub> </math></EquationSource> </InlineEquation> and propose a conjecture that an edge ideal <i>I</i>(<i>G</i>) has homological linear quotients if and only if <i>G</i> is co-chordal and <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\({{\,\textrm{H}\,}}_n^c\)</EquationSource> <EquationSource Format="MATHML"><math> <msubsup> <mrow> <mspace width="0.166667em" /> <mtext>H</mtext> <mspace width="0.166667em" /> </mrow> <mi>n</mi> <mi>c</mi> </msubsup> </math></EquationSource> </InlineEquation>-free for any <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(n\ge 6\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≥</mo> <mn>6</mn> </mrow> </math></EquationSource> </InlineEquation>. In this paper, we prove one direction of the conjecture. Moreover, we study possible patterns of pairs (<i>G</i>,&#xa0;<i>k</i>) of a co-chordal graph <i>G</i> and integer <i>k</i>, such that <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\({{\,\textrm{HS}\,}}_k(I(G))\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mrow> <mspace width="0.166667em" /> <mtext>HS</mtext> <mspace width="0.166667em" /> </mrow> <mi>k</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>I</mi> <mrow> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> has linear quotients.</p>

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Edge Ideals with Linear Quotients and Without Homological Linear Quotients

  • Trung Chau,
  • Kanoy Kumar Das,
  • Aryaman Maithani

摘要

A monomial ideal I is said to have homological linear quotients if, for each \(k\ge 0\) k 0 , the homological shift ideal \({{\,\textrm{HS}\,}}_k(I)\) HS k ( I ) has linear quotients. It is a well-known fact that if an edge ideal I(G) has homological linear quotients, then G is co-chordal. We construct a family of co-chordal graphs \(\{{{\,\textrm{H}\,}}_n^c\}_{n\ge 6}\) { H n c } n 6 and propose a conjecture that an edge ideal I(G) has homological linear quotients if and only if G is co-chordal and \({{\,\textrm{H}\,}}_n^c\) H n c -free for any \(n\ge 6\) n 6 . In this paper, we prove one direction of the conjecture. Moreover, we study possible patterns of pairs (Gk) of a co-chordal graph G and integer k, such that \({{\,\textrm{HS}\,}}_k(I(G))\) HS k ( I ( G ) ) has linear quotients.