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Cinzia Casagrande

Publications and source records attributed to Cinzia Casagrande.

17 recordsLinked to original sources
arXiv2026

On the geometry of Fano varieties with Lefschetz defect 2

Let X be a smooth, complex Fano variety, and delta(X) its Lefschetz defect. It is known that if delta(X) is at least 4, then X is isomorphic to a product SxT where S is a surface, whereas if delta(X)=3, X admits a del Pezzo fibration with a prescribed structure. In this paper, we study the case where delta(X)=2. Our first result is that there exists a sequence of flips X-->X' such that X' is smooth and admits a conic bundle structure X'->Y, with rho(X)-rho(Y)=2. The conic bundle factors as X'->X''->Y, where X'->X'' is a smooth blow-up, and f: X''->Y is again a conic bundle. Then we show that either f is smooth, or there exists a del Pezzo fibration g: X->T with rho(X)-rho(T)=3 and T smooth, admitting two possible generic fibers, and we describe the relative cone NE(g). Finally for one of the possible generic fibers we give a structure theorem for the del Pezzo fibration g; we show that T is Fano and that X can be reconstructed from T and a suitable divisor class on T. As an application, we construct a family of Fano 4-folds X exhibiting this behaviour.

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arXiv2024

Classifying Fano 4-folds with a rational fibration onto a 3-fold

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.

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arXiv2024

Fano 4-folds with $b_2>12$ are products of surfaces

Let X be a smooth, complex Fano 4-fold, and rho(X) its Picard number. We show that if rho(X)>12, then X is a product of del Pezzo surfaces. The proof relies on a careful study of divisorial elementary contractions f: X->Y such that the image S of the exceptional divisor is a surface, together with the author's previous work on Fano 4-folds. In particular, given f: X->Y as above, under suitable assumptions we show that S is a smooth del Pezzo surface with -K_S given by the restriction of -K_Y.

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arXiv2022

The Lefschetz defect of Fano varieties

This note is a short survey on the Lefschetz defect, an invariant of smooth Fano varieties that has been recently introduced; it is related to the Picard number rho(X) of X, and to the Picard number of prime divisors in X. We explain the definition of the Lefschetz defect delta(X) and its origin, the known results - mainly on the case delta(X)>1, and several examples, especially in dimensions 3 and 4. In particular, we explain how the results on the Lefschetz defect allow to recover the classification of Fano 3-folds with rho(X)>4, due to Mori and Mukai. We also review the known examples of Fano 4-folds with rho(X)>5, that are remarkably few: apart products and toric examples, there are only 6 known families, with rho(X) in {6,7,8,9}.

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arXiv2020

Classification of Fano 4-folds with Lefschetz defect 3 and Picard number 5

Let X be a smooth, complex Fano 4-fold, and rho(X) its Picard number. If X contains a prime divisor D with rho(X)-rho(D)>2, then either X is a product of del Pezzo surfaces, or rho(X)=5 or 6. In this setting, we completely classify the case where rho(X)=5; there are 6 families, among which one is new. We also deduce the classification of Fano 4-folds with rho(X)>4 with an elementary divisorial contraction sending a divisor to a curve.

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arXiv2019

Fano 4-folds with rational fibrations

We study (smooth, complex) Fano 4-folds X having a rational contraction of fiber type, that is, a rational map X-->Y that factors as a sequence of flips followed by a contraction of fiber type. The existence of such a map is equivalent to the existence of a non-zero, non-big movable divisor on X. Our main result is that if Y is not P^1 or P^2, then the Picard number rho(X) of X is at most 18, with equality only if X is a product of surfaces. We also show that if a Fano 4-fold X has a dominant rational map X-->Z, regular and proper on an open subset of X, with dim(Z)=3, then either X is a product of surfaces, or rho(X) is at most 12. These results are part of a program to study Fano 4-folds with large Picard number via birational geometry.

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arXiv2017

Fano 4-folds, flips, and blow-ups of points

In this paper we study smooth, complex Fano 4-folds X with large Picard number rho(X), with techniques from birational geometry. Our main result is that if X is isomorphic in codimension one to the blow-up of a smooth projective 4-fold Y at a point, then rho(X) is at most 12. We give examples of such X with Picard number up to 9. The main theme (and tool) is the study of fixed prime divisors in Fano 4-folds, especially in the case rho(X)>6, in which we give some general results of independent interest.

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arXiv2016

On the Fano variety of linear spaces contained in two odd-dimensional quadrics

In this paper we describe the geometry of the 2m-dimensional Fano manifold G parametrizing (m-1)-planes in a smooth complete intersection Z of two quadric hypersurfaces in the complex projective space P^{2m+2}, for m>0. We show that there are exactly 2^{2m+2} distinct isomorphisms in codimension one between G and the blow-up of P^{2m} at 2m+3 general points, parametrized by the 2^{2m+2} distinct m-planes contained in Z, and describe these rational maps explicitly. We also describe the cones of nef, movable and effective divisors of G, as well as their dual cones of curves. Finally, we determine the automorphism group of G. These results generalize to arbitrary even dimension the classical description of quartic del Pezzo surfaces (m=1).

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arXiv2015

Locally unsplit families of rational curves of large anticanonical degree on Fano manifolds

In this paper we address Fano manifolds X with a locally unsplit dominating family of rational curves of anticanonical degree equal to the dimension of X. We first observe that their Picard number is at most 3, and then we provide a classification of all cases with maximal Picard number. We also give examples of locally unsplit dominating families of rational curves whose varieties of minimal tangents at a general point is singular.

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arXiv2012

On the birational geometry of Fano 4-folds

We study the birational geometry of a Fano 4-fold X from the point of view of Mori dream spaces; more precisely, we study rational contractions of X. Here a rational contraction is a rational map f: X-->Y, where Y is normal and projective, which factors as a finite sequence of flips, followed by a surjective morphism with connected fibers. Such f is called elementary if the difference of the Picard numbers of X and Y is 1. We first give a characterization of non-movable prime divisors in X, when X has Picard number at least 6; this is related to the study of birational and divisorial elementary rational contractions of X. Then we study the rational contractions of fiber type on X which are elementary or, more generally, quasi-elementary. The main result is that the Picard number of X is at most 11 if X has an elementary rational contraction of fiber type, and 18 if X has a quasi-elementary rational contraction of fiber type.

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arXiv2007

Moduli of roots of line bundles on curves

We treat the problem of completing the moduli space for roots of line bundles on curves. Special attention is devoted to higher spin curves within the universal Picard scheme. Two new different constructions, both using line bundles on nodal curves as boundary points, are carried out and compared with pre-existing ones.

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arXiv2004

Moduli of Prym curves

Here we focus on the compactification of the moduli space of curves of genus g together with an unramified double cover, constructed by Arnaud Beauville in order to compactify the Prym mapping. We present an alternative description of it, inspired by the moduli space of spin curves of Maurizio Cornalba, and we discuss in detail its main features, both from a geometrical and a combinatorial point of view.

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arXiv2002

Combinatorial properties of stable spin curves

The geometry of the moduli space of stable spin curves is studied, with emphasis on its combinatorial properties. In this context, the standard graph theoretic framework is not just a book-keeping device: some purely combinatorial results are proved, having moduli theoretic applications. In particular, certain strata of the moduli space of stable curves are characterized by a (finite) set of integers, measuring the non-reducedness of the scheme of spin curves, and definable in purely graph-theoretical terms.

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arXiv2002

Contractible classes in toric varieties

Let X be a smooth, complete, toric variety. We study those curves C in X that are contractible, in the sense that there exists an equivariant morphism with connected fibers, with source X, that contracts exactly the irreducible curves that are numerically equivalent to a multiple of C. When X is projective, we compare contractible and extremal curves, and we show that every curve in X is numerically equivalent to a linear combination with positive integral coefficients of contractible curves.

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arXiv2001

Toric Fano varieties and birational morphisms

In this paper we study smooth toric Fano varieties using primitive relations and toric Mori theory. We show that for any irreducible invariant divisor D in a toric Fano variety X, we have $0\leqρ_X-ρ_D\leq 3$, for the difference of the Picard numbers of X and D. Moreover, if $ρ_X-ρ_D>0$ (with some additional hypotheses if $ρ_X-ρ_D=1$), we give an explicit birational description of X. Using this result, we show that when dim X=5, we have $ρ_X\leq 9$. In the second part of the paper, we study equivariant birational morphisms f whose source is Fano. We give some general results, and in dimension 4 we show that f is always a composite of smooth equivariant blow-ups. Finally, we study under which hypotheses a non-projective toric variety can become Fano after a smooth equivariant blow-up.

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arXiv2001

On the birational geometry of toric Fano 4-folds

We announce a factorization result for equivariant birational morphisms between toric 4-folds whose source is Fano: such a morphism is always a composite of blow-ups along smooth invariant centers. Moreover, we show with a counterexample that, differently from the 3-dimensional case, even if both source and target are Fano, the intermediate varieties can not be chosen Fano.

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