<p>For a projective variety <i>X</i>, we have the intersection complex <i>L</i>-classes <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(L_*(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>L</mi> <mrow> <mrow /> <mo>∗</mo> </mrow> </msub> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> defined by Goresky-MacPerson using cohomotopy and also the constant coefficient <i>L</i>-class <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(L^c_*(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msubsup> <mi>L</mi> <mrow> <mrow /> <mo>∗</mo> </mrow> <mi>c</mi> </msubsup> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> defined by applying an <i>L</i>-class transformation (or <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(T_{1*}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>T</mi> <mrow> <mn>1</mn> <mrow /> <mo>∗</mo> </mrow> </msub> </math></EquationSource> </InlineEquation>) to a cubic hyperresolution of <i>X</i>. These coincide if <i>X</i> is a <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\({\mathbb {Q}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="double-struck">Q</mi> </math></EquationSource> </InlineEquation>-homology manifold. We show that the two <i>L</i>-classes <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(L_*(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>L</mi> <mrow> <mrow /> <mo>∗</mo> </mrow> </msub> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(L^c_*(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msubsup> <mi>L</mi> <mrow> <mrow /> <mo>∗</mo> </mrow> <mi>c</mi> </msubsup> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> differ if they do by replacing <i>X</i> with an intersection of general hyperplane sections which has only <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\({\mathbb {Q}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="double-struck">Q</mi> </math></EquationSource> </InlineEquation>-homologically isolated singularities. Finding a good sufficient condition for the non-coincidence of <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(L_*(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>L</mi> <mrow> <mrow /> <mo>∗</mo> </mrow> </msub> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(L^c_*(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msubsup> <mi>L</mi> <mrow> <mrow /> <mo>∗</mo> </mrow> <mi>c</mi> </msubsup> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is thus reduced to the latter case, where a necessary and sufficient condition has been obtained in terms of the Hodge signatures of stalks of intersection complex in our previous paper. In the case of projective hypersurfaces having only isolated singularities, the difference between <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(L_*(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>L</mi> <mrow> <mrow /> <mo>∗</mo> </mrow> </msub> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(L^c_*(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msubsup> <mi>L</mi> <mrow> <mrow /> <mo>∗</mo> </mrow> <mi>c</mi> </msubsup> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is given by the Hodge signatures of the link cohomologies at singular points, and the Hodge signatures of the vanishing cohomologies give the difference between <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(L^c_*(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msubsup> <mi>L</mi> <mrow> <mrow /> <mo>∗</mo> </mrow> <mi>c</mi> </msubsup> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> and the virtual <i>L</i>-class of <i>X</i>, that is, the image by a retraction map of the <i>L</i>-class of a smooth deformation of <i>X</i> in an ambient smooth projective variety <i>Y</i> in the very ample case.</p>

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Constant coefficient and intersection complex L-classes of projective varieties

  • Javier Fernández de Bobadilla,
  • Irma Pallarés,
  • Morihiko Saito

摘要

For a projective variety X, we have the intersection complex L-classes \(L_*(X)\) L ( X ) defined by Goresky-MacPerson using cohomotopy and also the constant coefficient L-class \(L^c_*(X)\) L c ( X ) defined by applying an L-class transformation (or \(T_{1*}\) T 1 ) to a cubic hyperresolution of X. These coincide if X is a \({\mathbb {Q}}\) Q -homology manifold. We show that the two L-classes \(L_*(X)\) L ( X ) and \(L^c_*(X)\) L c ( X ) differ if they do by replacing X with an intersection of general hyperplane sections which has only \({\mathbb {Q}}\) Q -homologically isolated singularities. Finding a good sufficient condition for the non-coincidence of \(L_*(X)\) L ( X ) and \(L^c_*(X)\) L c ( X ) is thus reduced to the latter case, where a necessary and sufficient condition has been obtained in terms of the Hodge signatures of stalks of intersection complex in our previous paper. In the case of projective hypersurfaces having only isolated singularities, the difference between \(L_*(X)\) L ( X ) and \(L^c_*(X)\) L c ( X ) is given by the Hodge signatures of the link cohomologies at singular points, and the Hodge signatures of the vanishing cohomologies give the difference between \(L^c_*(X)\) L c ( X ) and the virtual L-class of X, that is, the image by a retraction map of the L-class of a smooth deformation of X in an ambient smooth projective variety Y in the very ample case.