Eleonora Anna RomanoUniversità degli Studi di Genova | UNIGE · Dipartimento di Matematica (DIMA)

22
Publications
385
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Publications

Publications (22)
Article
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We link small modifications of projective varieties with a C∗\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathbb {C}}^*$$\end{document}-action to their GIT quotien...
Preprint
In this paper we study varieties admitting torus actions as geometric realizations of birational transformations. We present an explicit construction of these geometric realizations for a particular class of birational transformations, and study some of their geometric properties, such as their Mori, Nef and Movable cones.
Article
Let X be a smooth, complex Fano variety, and δX its Lefschetz defect. By [4], if δX≥4, then X≅S×T, where dim⁡T=dim⁡X−2. In this paper we prove a structure theorem for the case where δX=3. We show that there exists a smooth Fano variety T with dim⁡T=dim⁡X−2 such that X is obtained from T with two possible explicit constructions; in both cases there...
Preprint
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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 dim T=dim X-2. In this paper we prove a structure theorem for the case where delta(X)=3. We show that there exists a smooth Fano variety T with dim T=dim X-2 such that X is obtained fro...
Preprint
A geometric realization of a birational map $\psi$ among two complex projective varieties is a variety $X$ endowed with a $\mathbb{C}^*$-action inducing $\psi$ as the natural birational map among two extremal geometric quotients. In this paper we study geometric realizations of some classic birational maps --inversion maps, special Cremona transfor...
Article
Let X be a smooth, complex Fano 4-fold, and ρX its Picard number. If X contains a prime divisor D with ρX−ρD>2, then either X is a product of del Pezzo surfaces, or ρX=5,6. In this setting, we completely classify the case where ρX=5; there are 6 families, among which one is new. We also deduce the classification of Fano 4-folds with ρX≥4 with an el...
Preprint
Starting from $\mathbb{C}^*$-actions on complex projective varieties, we construct and investigate birational maps among the corresponding extremal fixed point components. We study the case in which such birational maps are locally described by toric flips, either of Atiyah type or so called non-equalized. We relate this notion of toric flip with t...
Preprint
We link small modifications of projective varieties with a ${\mathbb C}^*$-action to their GIT quotients. Namely, using flips with centers in closures of Bia{\l}ynicki-Birula cells, we produce a system of birational equivariant modifications of the original variety, which includes those on which a quotient map extends from a set of semistable point...
Article
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We prove LeBrun–Salamon conjecture in the following situation: if X is a contact Fano manifold of dimension 2n+1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$2n+1$$\e...
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Let X be a complex projective manifold, L an ample line bundle on X, and assume that we have a ℂ* action on (X;L). We classify such triples (X; L;ℂ*) for which the closure of a general orbit of the ℂ* action is of degree ≤ 3 with respect to L and, in addition, the source and the sink of the action are isolated fixed points, and the ℂ* action on the...
Preprint
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-fol...
Preprint
We prove LeBrun--Salamon conjecture in the following situation: if $X$ is a contact Fano manifold of dimension $2n+1$ whose group of automorphisms is reductive of rank $\geq \max(2,(n-3)/2)$ then $X$ is the adjoint variety of a simple group. The rank assumption is fulfilled not only by the three series of classical linear groups but also by almost...
Preprint
In this paper we classify varieties of Picard number two having two projective bundle structures of any relative dimension, under the assumption that these structures are mutually uniform. As an application we prove the Campana--Peternell conjecture for varieties of Picard number one admitting $\mathbb C^*$-actions of a certain kind.
Preprint
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In this paper we study smooth projective varieties and polarized pairs with an action of a one dimensional complex torus. As a main tool, we define birational geometric counterparts of these actions, that, under certain assumptions, encode the information necessary to reconstruct them. In particular, we consider some cases of actions of low complex...
Article
We find a characterization for Fano 4-folds $X$ with Lefschetz defect $\delta _{X}=3$: besides the product of two del Pezzo surfaces, they correspond to varieties admitting a conic bundle structure $f\colon X\to Y$ with $\rho _{X}-\rho _{Y}=3$. Moreover, we observe that all of these varieties are rational. We give the list of all possible targets o...
Preprint
We discuss the flatness property of some fiber type contractions of complex smooth projective varieties of arbitrary dimensions. We relate the flatness of some morphisms having one-dimensional fibers with their conic bundles structures, also in the general case in which some mild singularities of the varieties are admitted.
Preprint
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Let $X$ be a complex projective manifold, $L$ an ample line bundle on $X$ and $H=\mathbb{C}^*$ an algebraic torus acting on $(X,L)$. We classify such triples $(X,L,H)$ for which the closure of a general orbit of the action of $H$ is of degree $\leq 3$ with respect to $L$ and, in addition, the source and the sink of the action are isolated fixed poi...
Article
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Let $X$ be a complex projective Fano $4$-fold. Let $D\subset X$ be a prime divisor. Let us consider the image $\mathcal{N}_{1}(D,X)$ of $\mathcal{N}_{1}(D)$ in $\mathcal{N}_{1}(X)$ through the natural push-forward of one-cycles. Let us consider the following invariant of $X$ given by \$\delta_{X}:=\max\{\operatorname{codim} \mathcal{N}_{1}(D,X)\;|\;...
Article
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We give the first examples of flat fiber type contractions of Fano manifolds onto varieties that are not weak Fano, and we prove that these morphisms are Fano conic bundles. We also review some known results about the interaction between the positivity properties of anticanonical divisors of varieties of contractions.
Article
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We study a particular kind of fiber type contractions between complex, projective, smooth varieties f:X->Y, called Fano conic bundles. This means that X is a Fano variety, and every fiber of f is isomorphic to a plane conic. Denoting by rho_{X} the Picard number of X, we investigate such contractions when rho_{X}-rho_{Y} is greater than 1, called n...

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