Abstract
We consider the union of certain irreducible components of cohomological support loci of the canonical bundle, which we call standard. We prove a structure theorem about them and single out some particular cases, recovering and improving results of Beauville and Chen–Jiang. Finally, as an example of application, we extend to compact Kahler manifolds the classification of smooth complex projective varieties with \(p_1(X)=1\), \(p_3(X)=2\) and \(q(X)=\dim X\).
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Notes
Proof: let \(X_g\) be a general fiber of g. Then g induces a map \(a:X_g\rightarrow K:=\ker (\mathrm {Alb}\, X\rightarrow \mathrm {Alb}\, Y)\) whose image spans K. Hence the homomorphism \(\mathrm{Pic}^0 K\rightarrow \mathrm{Pic}^0 F\) has finite kernel. Then the exact sequence (1.5) follows from the dualization of the exact sequence \(0\rightarrow K\rightarrow \mathrm {Alb}\, X\rightarrow \mathrm {Alb}\, Y\rightarrow 0\).
The holomorphic Euler characteristic \(\chi (K_{\widetilde{Y}})\) does not depend on the particular resolution \(\widetilde{Y}\) considered. Since one can choose a \(\widetilde{Y}\) which is Kahler (see e.g. [5, 1.9]), \(\chi (K_{\widetilde{Y}})\ge 0\) by generic vanishing (see below).
If \(V^i(K_X)\) is empty or zero-dimensional for all i we define \(p=-\infty \).
Strictly speaking the proof of this application uses only the well known theorem of Beauville mentioned above, which is now a particular case of Theorem C. However we included it in this paper because it is suggestive about the possible use of Theorems A and C when dealing with this sort of problems.
The surjective homorphisms with connected fibres \(\pi _k:A\rightarrow B_k\) are not uniquely determined. However one can arrange them in such a way that \(\pi _k\) factorizes trough \(\pi _h\) if \(\pi _k^*\mathrm{Pic}^0 B_k\) is contained \(\pi _h^*\mathrm{Pic}^0 B_h\).
In brief: one can define more generally loci \(V^i_{m}(K_X\otimes P_\eta )=\{\alpha \in \mathrm{Pic}^0 X\>|\> h^i(K_X\otimes P_\eta \otimes P_\alpha )\ge m\}\) and the Theorems of Green-Lazarsfeld and Simpson-Wang prove as well that all components \(V^i_{m}(K_X\otimes P_\eta )\) are translates of subtori by points of finite order. Then one proves, using Kollár’s decomposition, that a component of \(V^r(R^i g_*(K_X\otimes P_\eta ))\) is also a component of \(V^{r+i}_m(K_X\otimes P_\eta )\) for some m.
We recall (see the footnote to Remark 3.5) that, unlike the homomorphisms \(\pi _{B}\), the subtori T are uniquely determined by the Chen–Jiang decomposition.
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I am very indebted to Zhi Jiang for pointing out some gaps and errors in a previous version of this paper.
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Dedicated to Philippe Ellia on the occasion of his 60th birthday.
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Pareschi, G. Standard canonical support loci. Rend. Circ. Mat. Palermo, II. Ser 66, 137–157 (2017). https://doi.org/10.1007/s12215-016-0269-3
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DOI: https://doi.org/10.1007/s12215-016-0269-3