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Gianfranco Casnati

Publications and source records attributed to Gianfranco Casnati.

36 records · Page 2Linked to original sources
arXiv2015

Irreducibility of the Gorenstein loci of Hilbert schemes via ray families

We analyse the Gorenstein locus of the Hilbert scheme of $d$ points on $\mathbb{P}^n$ i.e. the open subscheme parameterising zero-dimensional Gorenstein subschemes of $\mathbb{P}^n$ of degree $d$. We give new sufficient criteria for smoothability and smoothness of points of the Gorenstein locus. In particular we prove that this locus is irreducible when $d\leq 13$ and find its components when $d = 14$. The proof is relatively self-contained and it does not rely on a computer algebra system. As a by--product, we give equations of the fourth secant variety to the $d$-th Veronese reembedding of $\mathbb{P}^n$ for $d\geq 4$.

math.AG↗
arXiv2015

Rank two aCM bundles on the del Pezzo fourfold of degree $6$ and its general hyperplane section

In the present paper we completely classify locally free sheaves of rank $2$ with vanishing intermediate cohomology modules on the image of the Segre embedding $\mathbb{P}^2\times\mathbb{P}^2\subseteq\mathbb{P}^8$ and its general hyperplane sections. Such a classification extends similar already known results regarding del Pezzo varieties with Picard numbers $1$ and $3$ and dimension at least $3$.

math.AG↗
arXiv2014

Rank two aCM bundles on the del Pezzo threefold with Picard number 3

Let k be an algebraically closed field of characteristic 0. A del Pezzo threefold F with maximal Picard number is isomorphic to P^1xP^1xP^1, where P^1 is the projective line over k. In the present paper we completely classify locally free sheaves of rank 2 with vanishing intermediate cohomology over such an F. Such a classification extends similar results proved by E. Arrondo and L. Costa regarding del Pezzo threefolds with Picard number 1.

math.AG↗
arXiv2014

A structure theorem for 2-stretched Gorenstein algebras

In this paper we study the isomorphism classes of local, Artinian, Gorenstein k-algebras A whose maximal ideal M satisfies dim_k(M^3/M^4)=1 by means of Macaulay's inverse system generalizing a recent result by J. Elias and M.E. Rossi. Then we use such results in order to complete the description of the singular locus of the Gorenstein locus of the punctual Hilbert scheme of degree 11.

math.AC↗
arXiv2014

On the rationality of Poincaré series of Gorenstein algebras via Macaulay's correspondence

Let $A$ be a local Artinian Gorenstein ring with algebraically closed residue field $A/{\frak M}=k$ of characteristic 0, and let $P_A(z) := \sum_{p=0}^{\infty} ({\mathrm{ Tor}}_p^A(k,k))z^p $ be its Poincaré series. We prove that $P_A(z)$ is rational if either $\dim_k({{\frak M}^2/{\frak M}^3}) \leq 4 $ and $ \dim_k(A) \leq 16,$ or there exist $m\leq 4$ and $c$ such that the Hilbert function $H_A(n)$ of $A$ is equal to $ m$ for $n\in [2,c]$ and equal to 1 for $n > c$. The results are obtained thanks to a decomposition of the apolar ideal $\mathrm {Ann}(F)$ when $F=G+H$ and $G$ and $H$ belong to polynomial rings in different variables.

math.AC↗
arXiv2011

Birational properties of some moduli spaces related to tetragonal curves of genus 7

Let M_{7,n} be the (coarse) moduli space of smooth curves of genus 7 with n marked points defined over the complex field. We denote by M^1_{7,n;4} the locus of points inside M_{7,n} representing curves carrying a g^1_4. It is classically known that M^1_{7,n;4} is irreducible of dimension 17+n. We prove in this paper that M^1_{7,n;4} is rational for 0<= n <= 11.

math.AG↗
arXiv2010

Poincaré series and deformations of Gorenstein local algebras with low socle degree

Let $K$ be an algebraically closed field of characteristic $0$, and let $A$ be an Artinian Gorenstein local commutative and Noetherian $K$--algebra, with maximal ideal $M$. In the present paper we prove a structure theorem describing such kind of $K$--algebras satisfying $M^4=0$. We use this result in order to prove that such a $K$--algebra $A$ has rational Poincaré series and it is always smoothable in any embedding dimension, if $\dim_K M^2/M^3 \le 4$. We also prove that the generic Artinian Gorenstein local $K$--algebra with socle degree three has rational Poincaré series, in spite of the fact that such algebras are not necessarily smoothable.

math.AC↗
arXiv2010

Irreducibility of the Gorenstein locus of the punctual Hilbert scheme of degree 10

Let $k$ be an algebraically closed field of characteristic 0 and let $H_G(d,N)$ be the open locus of the Hilbert scheme $H(d,N)$ corresponding to Gorenstein subschemes of degree $d$ in the projective N-space. We proved in a previous paper that $H_G(d,N)$ is irreducible for $d\le9$ and $N\ge1$. In the present paper we prove that also $H_G(10,N)$ is irreducible for each $N\ge1$, giving also a complete description of its singular locus.

math.AG↗
arXiv2010

Canonical curves with low apolarity

Let $k$ be an algebraically closed field and let $C$ be a non--hyperelliptic smooth projective curve of genus $g$ defined over $k$. Since the canonical model of $C$ is arithmetically Gorenstein, Macaulay's theory of inverse systems allows to associate to $C$ a cubic form $f$ in the divided power $k$--algebra $R$ in $g-2$ variables. The apolarity of $C$ is the minimal number $t$ of linear form in $R$ needed to write $f$ as sum of their divided power cubes. It is easy to see that the apolarity of $C$ is at least $g-2$ and P. De Poi and F. Zucconi classified curves with apolarity $g-2$ when $k$ is the complex field. In this paper, we give a complete, characteristic free, classification of curves $C$ with apolarity $g-1$ (and $g-2$).

math.AG↗
arXiv2008

The Poincaré series of a local Gorenstein ring of multiplicity up to 10 is rational

Let $R$ be a local, Gorenstein ring with algebraically closed residue field $k$ of characteristic 0 and let $P_R(z):=\sum_{p=0}^{\infty}\dim_k(\tor_p^R(k,k))z^p$ be its Poincaré series. We compute $P_R$ when $R$ belongs to a particular class defined in the introduction, proving its rationality. As a by--product we prove the rationality of $P_R$ for all local, Gorenstein rings of multiplicity at most 10.

math.AC↗
arXiv2008

On the Gorenstein locus of some punctual Hilbert schemes

Let $k$ be an algebraically closed field and let $\Hilb_{d}^{G}(\p{N})$ be the open locus of the Hilbert scheme $\Hilb_{d}(\p{N})$ corresponding to Gorenstein subschemes. We prove that $\Hilb_{d}^{G}(\p{N})$ is irreducible for $d\le9$, we characterize geometrically its singularities for $d\le 8$ and we give some results about them when $d=9$ which give some evidence to a conjecture on the nature of the singular points in $\Hilb_{d}^{G}(\p{N})$.

math.AG↗
arXiv2007

On the rationality of moduli spaces of pointed curves

Motivated by several recent results on the geometry of the moduli spaces $\bar{\Cal M}_{g,n}$ of stable curves of genus $g$ with $n$ marked points, here we determine their birational structure for small values of $g$ and $n$ by exploiting suitable plane models of the general curve. More precisely, we show that ${\Cal M}_{g,n}$ is rational for $g=2$ and $1\le n\le 12$, $g=3$ and $1\le n\le 14$, $g=4$ and $1\le n\le 15$, $g=5$ and $1\le n\le 12$.

math.AG↗
arXiv2007

On the birational geometry of moduli spaces of pointed curves

We prove that the moduli space ${\Cal M}_{g,n}$ of smooth curves of genus $g$ with $n$ marked points is rational for $g=6$ and $1 \le n \le 8$, and it is unirational for $g=8$ and $1 \le n \le 11$, $g=10$ and $1 \le n \le 3$, $g=12$ and $n = 1$.

math.AG↗
arXiv2006

On some Gorenstein loci in $\Hilb_{6}(\p 4)$

Let k be an algebraically closed field and let HaG(d) be the open locus inside H(d) (the Hilbert scheme of 0-dimensional length d subschemes of the projective (d-2)-space over k) corresponding to arithmetically Gorenstein subschemes. We prove the irreducibility and characterize the singularities of HaG(6). In order to achieve these results we also classify all Artinian, Gorenstein, not necessarily graded, k-algebras up to degree 6. Moreover we describe the loci in HaG(6) obtained via some geometric construction. Finally we prove the obstructedness of some families of points in HaG(d) for each d>5.

math.AG↗
arXiv2001

Quadratic sheaves and self-linkage

Generalizing an old result proved by P. Rao [see MR 83i:14025] for arithmetically Cohen-Macaulay, self-linked subschemes of codimension 2 in the projective n-space P, we give a characterization of self-linked pure subschemes of codimension 2 in P satisfying a necessary parity condition, in characteristic different from 2. We make use of the theory of quadratic sheaves described in a previous paper of the authors [see MR 99h:14044].

math.AG↗
arXiv1996

Even sets of nodes are bundle symmetric

Let k be an algebraically closed field of characteristic p different from 2, and let F be a nodal surface of degree d in the projective 3-space P over k (i.e. the singularities of F are only ordinary quadratic, nodes for short). Let N be a subset of the set of nodes of F: then N is said to be n/2-even for n=0,1 if the following condition (*) holds. Namely, let F' be a minimal resolution of the singularities of F, let N' be the inverse image of N in F', let H be the inverse image of a plane section of F. Then N is n/2 if: (*) the class of N'+nH is 2-divisible in Pic(F'). In this paper we prove the following characterization of even sets, which has been conjectured by W. Barth and the second author in 1979. Let F be as above. Then the n/2-even set of nodes N on F are exactly the bundle symmetric sets, i.e. the degeneracy loci of symmetric maps s:E*(-d-n)->E of a suitable locally free sheaf E over P (i.e. F is the locus where rk(s)<rkE, N is the locus where rk(s)=rkE-2). We also give a classification of even sets in degree d=4,5.

alg-geom↗