Classifying Fano 4-folds with a rational fibration onto a 3-fold
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arXiv
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| Format: | Preprint |
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2024
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| _version_ | 1866916458940858368 |
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| author | Casagrande, Cinzia Secci, Saverio Andrea |
| author_facet | Casagrande, Cinzia Secci, Saverio Andrea |
| contents | We study smooth, complex Fano 4-folds X with a rational contraction onto a 3-fold, namely a rational map X-->Y that factors as a sequence of flips X-->X' followed by a surjective morphism X'->Y with connected fibers, where Y is normal, projective, and dim Y=3. We show that if X has a rational contraction onto a 3-fold and X is not a product of del Pezzo surfaces, then the Picard number rho(X) of X is at most 9; this bound is sharp. As an application, we show that every Fano 4-fold X with rho(X)=12 is isomorphic to a product of surfaces, thus improving the result by the first named author that shows the same for rho(X)>12. We also give a classification result for Fano 4-folds X, not products of surfaces, having a "special" rational contraction X-->Y with dim Y=3, rho(X)-rho(Y)=2, and rho(X)>6; we show that there are only three possible families. Then we prove that the first family exists if rho(X)=7, and that the second family exists if and only if rho(X)=7. This provides the first examples of Fano 4-folds with rho(X)>6 different from products of del Pezzo surfaces and from the Fano models of the blow-up of P^4 in points. We also construct three new families with rho(X)=6. Finally we show that if a Fano 4-fold X has Lefschetz defect delta(X)=2, then rho(X) is at most 6; this bound is again sharp. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2408_10337 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Classifying Fano 4-folds with a rational fibration onto a 3-fold Casagrande, Cinzia Secci, Saverio Andrea Algebraic Geometry 14J45, 14J35, 14E30 We study smooth, complex Fano 4-folds X with a rational contraction onto a 3-fold, namely a rational map X-->Y that factors as a sequence of flips X-->X' followed by a surjective morphism X'->Y with connected fibers, where Y is normal, projective, and dim Y=3. We show that if X has a rational contraction onto a 3-fold and X is not a product of del Pezzo surfaces, then the Picard number rho(X) of X is at most 9; this bound is sharp. As an application, we show that every Fano 4-fold X with rho(X)=12 is isomorphic to a product of surfaces, thus improving the result by the first named author that shows the same for rho(X)>12. We also give a classification result for Fano 4-folds X, not products of surfaces, having a "special" rational contraction X-->Y with dim Y=3, rho(X)-rho(Y)=2, and rho(X)>6; we show that there are only three possible families. Then we prove that the first family exists if rho(X)=7, and that the second family exists if and only if rho(X)=7. This provides the first examples of Fano 4-folds with rho(X)>6 different from products of del Pezzo surfaces and from the Fano models of the blow-up of P^4 in points. We also construct three new families with rho(X)=6. Finally we show that if a Fano 4-fold X has Lefschetz defect delta(X)=2, then rho(X) is at most 6; this bound is again sharp. |
| title | Classifying Fano 4-folds with a rational fibration onto a 3-fold |
| topic | Algebraic Geometry 14J45, 14J35, 14E30 |
| url | https://arxiv.org/abs/2408.10337 |