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Nicolae Manolache

Publications and source records attributed to Nicolae Manolache.

14 recordsLinked to original sources
arXiv2021

On the vector bundles from Chang and Ran's proof of the unirationality of $\mathcal{M}_g$, $g \leq 13$

We combine the idea of Chang and Ran [Invent. Math. 76 (1984), 41-54] of using monads of vector bundles on the projective 3-space to prove the unirationality of the moduli spaces of curves of low genus with our classification of globally generated vector bundles with small first Chern class $c_1$ on the projective 3-space to get an alternative argument for the unirationality of the moduli spaces of curves of degree at most 13 (based on the general framework of Chang and Ran).

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arXiv2020

Globally generated vector bundles with $c_1 = 5$ on $\mathbb{P}^n$, $n \geq 4$

We complete the classification of globally generated vector bundles with small $c_1$ on projective spaces by treating the case $c_1 = 5$ on $\mathbb{P}^n$, $n \geq 4$ (the case $c_1 \leq 3$ has been considered by Sierra and Ugaglia, while the cases $c_1 = 4$ on any projective space and $c_1 = 5$ on $\mathbb{P}^2$ and $\mathbb{P}^3$ have been studied in two of our previous papers). It turns out that there are very few indecomposable bundles of this kind: besides some obvious examples there are, roughly speaking, only the (first twist of the) rank 5 vector bundle which is the middle term of the monad defining the Horrocks bundle of rank 3 on $\mathbb{P}^5$, and its restriction to $\mathbb{P}^4$. We recall, in an appendix, from our preprint [arXiv:1805.11336], the main results allowing the classification of globally generated vector bundles with $c_1 = 5$ on $\mathbb{P}^3$. Since there are many such bundles, a large part of the main body of the paper is occupied with the proof of the fact that, except for the simplest ones, they do not extend to $\mathbb{P}^4$ as globally generated vector bundles.

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arXiv2018

Globally generated vector bundles with $c_1 = 5$ on $\mathbb{P}^3$

We provide a classification of globally generated vector bundles with $c_1 = 5$ on the projective 3-space. The classification is complete (except for one case) but not as detailed as the corresponding classification in the case $c_1 = 4$ from our paper [Memoirs A.M.S., Vol. 253, No. 1209 (2018), also arXiv:1305.3464]. We determine, at least, the pairs of integers $(a , b)$ for which there exist globally generated vector bundles on the projective 3-space with Chern classes $c_1 = 5$, $c_2 = a$, $c_3 = b$ (except for the case $(12 , 0)$ and the complementary case $(13 , 5)$ which remain undecided), we describe the Horrocks monads of these vector bundles and we organize them into several families with irreducible bases. We use some of the results from our paper [arXiv:1502.05553] (for which we give, however, a direct self-contained proof in one of the appendices of the present paper) to reduce the problem to the classification of stable rank 3 vector bundles $F$ with $c_1(F) = -1$, $2 \leq c_2(F) \leq 4$, having the property that $F(2)$ is globally generated. We use, then, the spectrum of such a bundle to get the necessary cohomological information. Some of the constructions appearing in the present paper are used (and reproduced, for the reader's convenience) in another paper of ours [arXiv:1711.06060] in which we provide an alternative to Chang and Ran's proof of the unirationality of the moduli spaces of curves of degree at most 13 from [Invent. Math. 76 (1984), 41--54].

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arXiv2016

Globally Generated Vector Bundles on P^n with c_1=4

We classify globally generated vector bundles on the projective n-space with first Chern class = 4. This extends previous results for first Chern class at most 3, namely for 2 of Sierra and Ugaglia [J. Pure Appl. Algebra 213 (2009), 2141-2146] and for 3 of Anghel and Manolache [arXiv:1202.6261] and, independently, of Sierra and Ugaglia [arXiv:1203.0185]. It turns out that the case first Chern class = 4 is much more involved than the previous cases, especially on the projective 3-space. Among the bundles appearing in our classification one can find the Sasakura rank 3 vector bundle on the projective 4-space (suitably twisted). In the new version Sections 1, 2, 4. 5, 6 and 7 have been rewritten and some arguments and the presentation have been, hopefully, improved.

math.AG↗
arXiv2016

Locally Cohen-Macaulay space curves defined by cubic equations and globally generated vector bundles

We classify globally generated vector bundles with first Chern class $c_1$ at least 4 on the projective 3-space with the property that $E(-c_1+3)$ has a non-zero global section. This (seemingly) technical result allows one to reduce the classification of globally generated vector bundles with $c_1$ at most 7 on the projective 3-space to the classification of stable rank-2 reflexive sheaves with the same properties. The proof is based on a description of the monads of all locally Cohen-Macaulay space curves defined by cubic equations. We extend then this kind of classification to higher dimensional projective spaces. We use this extension to recuperate quickly the classification of globally generated vector bundles with $c_1=4$ on the projective $n$-space for $n$ at least 4, which is part of the main result of our previous paper [arxiv:1305.3464]. We provide, in the appendices to the paper, graded free resolutions for the homogeneous ideals and for the graded structural algebras of all non-reduced locally Cohen-Macaulay space curves of degree at most 4.

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arXiv2016

Four generated 4-instantons

We show that there exist mathematical 4-instanton bundles F on the projective 3-space such that F(2) is globally generated (by four global sections). This is equivalent to the existence of elliptic space curves of degree 8 defined by quartic equations. There is a (possibly incomplete) intersection theoretic argument for the existence of such curves in D'Almeida [Bull. Soc. Math. France 128 (2000), 577-584] and another argument, using results of Mori [Nagoya Math. J. 96 (1984), 127-132], in Chiodera and Ellia [Rend. Istit. Univ. Trieste 44 (2012), 413-422]. Our argument is quite different. We prove directly the former fact, using the method of Hartshorne and Hirschowitz [Ann. Scient. Ec. Norm. Sup. (4) 15 (1982), 365-390] and the geometry of five lines in the projective 3-space.

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arXiv2012

Globally Generated Vector Bundles on P^n with c_1=3

One classifies the globally generated vector bundles on P^n (n \not = 3) with the first Chern class c_1 = 3. The case n = 3 is treated in arXiv:1202.5988 [math.AG]. The case c_1 = 2 was treated by J.C. Sierra and L. Ugaglia (see References), the case c_1 = 3, rank = 2 is settled by S. Huh (see References), the case rank = 2, c_1 \le 5 is studied by L. Chiodera and Ph. Ellia (see References).

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arXiv2012

Globally Generated Vector Bundles on P^3 with c_1=3

One classifies the globally generated vector bundles on P^3 with the first Chern class c_1=3. The case c_1=2 on P^n was done by J.C. Sierra and L. Ugaglia (see the References) and the case c_1=3, rank=2 on P^n was done by S. Huh (see the References).

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arXiv2005

Linkage Extensions

Given two equidimensional Cohen-Macaulay local rings of the same dimension, one shows that a simultaneous extension of each of them by a dualizing module of the other is Gorenstein. This generalizes a theorem of Fossum. The geometrical analogue is also considered. The pairs of double lines in the projective space which are algebraically linked are classified.

math.AG↗
arXiv2003

Cohen-Macaulay Nilpotent Schemes

The first part of this paper, written mainly for nonspecialists, is a short and partial survey about the construction and classification of nilpotent Cohen-Macaulay scheme structures on a scheme "less nilpotent"(e.g. a smooth variety) as support. The second part presents some old and new examples about degree 4 curves in the projective space and inexistence of certain vector bundles.

math.AG↗
arXiv2000

Double Rational Normal Curves with Linear Syzygies

One describes those double structures on rational normal curves which are defined scheme theoretically by quadratic equations and have linear syzygies, generalizing this way the double line in the plane

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