Nitin Nitsure
Let $S$ be a noetherian scheme, and let $E$ be a coherent sheaf on it. We define a group-valued contravariant functor $GL_E$ on $S$-schemes by associating to any $S$-scheme $T$ the group $GL_E(T)$ of all linear automorphisms of the pullback of $E$ to $T$. This functor is clearly a sheaf in the fpqc topology. We prove that $GL_E$ is representable by a group-scheme over $S$ if and only if the sheaf $E$ is locally free.
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Recently N. Nitsure showed that for a coherent sheaf F on a noetherian scheme the automorphism functor Aut_F is representable if and only if F is locally free. Here we remove the noetherian hypothesis and show that the same result holds for the endomorphism functor End_F even if one asks for representability by an algebraic space.
Let $X$ be a projective scheme over a noetherian base scheme $S$, and let $F$ be a coherent sheaf on $X$. For any coherent sheaf $E$ on $X$, consider the set-valued contravariant functor $Hom_{E,F}$ on $S$-schemes, defined by $Hom_{E,F}(T) = Hom(E_T,F_T)$ where $E_T$ and $F_T$ are the pull-backs of $E$ and $F$ to $X_T = X\times_S T$. A basic result of Grothendieck ([EGA] III 7.7.8, 7.7.9) says that if $F$ is flat over $S$ then $Hom_{E,F}$ is representable for all $E$. We prove the converse of the above, in fact, we show that if $L$ is a relatively ample line bundle on $X$ over $S$ such that th…
A Beauville-Laszlo-type descent theorem for locally Noetherian schemes
该研究针对局部诺特概形的凝聚层下降问题，通过将粘合空间定义为adic空间而非Berkovich空间，推广了已有下降定理的适用范围。
We prove the following result of V. Voevodsky. If $S$ is a finite dimensional noetherian scheme such that $S=\cup_α\Spec(R_α)$ for {\em countable} rings $R_α$, then the stable motivic homotopy category over $S$ satisfies Brown representability.
Representability of the automorphism group of finitely generated vertex algebras
研究自由代数的自同构群，证明有限生成顶点代数在其noetherian环上的自同构群为仿射群方案。
Relative augmented base loci on noetherian schemes
本文在诺特概形上证明可逆层的相对增广基轨迹等于相对例外轨迹，并给出推广 Keel 结果的半丰富性判据。
Relative representability and parahoric level structures
本文基于$v$-sheaf理论修正的形式方案可表示性准则，研究局部shtukas上的paraoric层次结构模空间，并探讨超专层次下Shimura品种积分模型的局部可表示性及forgetful映射性质。
We give some properties (cancellation, representability, stratification) of the sheaf R^i f_* G for an affine relative curve f:U -> S admitting a smooth compactification and G a solvable group.
We prove that on separated algebraic surfaces every coherent sheaf is a quotient of a locally free sheaf. This class contains many schemes that are neither normal, reduced, quasiprojective or embeddable into toric varieties. Our methods extend to arbitrary $2$-dimensional schemes that are proper over a noetherian ring.
This is the second of two papers on the injective spectrum of a right noetherian ring. In the prequel, we considered the injective spectrum as a topological space associated to a ring (or, more generally, a Grothendieck category), which generalises the Zariski spectrum. We established some results about the topology and its links with Krull dimension, and computed a number of examples. In the present paper, which can largely be read independently of the first, we extend these results by defining a sheaf of rings on the injective spectrum and considering sheaves of modules over this structure s…
Let $(\mathcal C,\otimes,1)$ be an abelian symmetric monoidal category satisfying certain conditions and let $X$ be a scheme over $(\mathcal C,\otimes,1)$ in the sense of Toën and Vaquié. In this paper we show that when $X$ is quasi-compact and semi-separated, any quasi-coherent sheaf on $X$ may be expressed as a directed colimit of its finitely generated quasi-coherent submodules. Thereafter, we introduce a notion of "field objects" in $(\mathcal C,\otimes,1)$ that satisfy several properties similar to those of fields in usual commutative algebra. Finally we show that the points of a Noetheri…
We classify the prelocalizing subcategories of the category of quasi-coherent sheaves on a locally noetherian scheme. In order to give the classification, we introduce the notion of a local filter of subobjects of the structure sheaf. The essential part of the argument is given as results on a Grothendieck category with certain properties. We also classify the localizing subcategories, the closed subcategories, and the bilocalizing subcategories in terms of filters.
We prove the following criterion for the pro-representability of the deformation cohomology of a commutative formal Lie group. Let f be a flat and separated morphism between noetherian schemes. Assume that the target of f is flat over the integers. For a commutative formal Lie group E, we have the deformation cohomology of f with coefficients in E at our disposal. If the higher direct images of the tangent space of E are locally free and of finite rank then the deformation cohomology is pro-representable by a commutative formal Lie group.
This a first step to develop a theory of smooth, etale and unramified morphisms between noetherian formal schemes. Our main tool is the complete module of differentials, that is a coherent sheaf whenever the map of formal schemes is of pseudo finite type. Among our results we show that these infinitesimal properties of a map of usual schemes carry over into the completion with respect to suitable closed subsets. We characterize unramifiedness by the vanishing of the module of differentials. Also we see that a smooth morphism of noetherian formal schemes is flat and its module of differentials …
Let $X \overset{f}\longrightarrow S$ be a morphism of Noetherian schemes, with $S$ reduced. For any closed subscheme $Z$ of $X$ finite over $S$, let $j$ denote the open immersion $X\setminus Z \hookrightarrow X$. Kollár asked whether for any coherent sheaf $\mathcal F$ on $X\setminus Z$ and any index $r\geq 1$, the sheaf $f_*(R^rj_*\mathcal F)$ is generically free on $S$ and commutes with base change. We answer this affirmatively, by proving a related statement about local cohomology: Let $ R$ be Noetherian algebra over a Noetherian domain $A$, and let $I \subset R$ be an ideal such that $ R/I…
A blowup criterion for regularity
本文证明了诺特整环正则等价于沿所有爆破的结构层导出前推为完美复形，并构造了局部环中剩余域平移作为显式爆破导出前推的直和项。
Regularity is bounded on a quasi-excellent Noetherian scheme
本文证明拟优良诺特概形上切锥的梯度Betti表只有有限种可能，从而正则性有界。
We define a derived enhancement of the classical quot functor of quotients associated to a coherent sheaf on a nonsingular quasiprojective variety. We prove its representability and show that it has the expected tangent complex. The derived quot scheme of points can be covered by affine charts obtained as spectra of commutative graded differential algebras (cdgas) and we compute an example. As a demonstration of the usefulness of this presentation, we write down an explicit shifted form on this example and conjecture that it agrees with the shifted symplectic structure on the derived stack of …
We show that a closed subscheme of a given locally noetherian scheme can be constructed by prescribing it germs at all points of the ambient scheme in a manner consistent with specialization of points, provided the resulting set of all associated points of all the germs is locally finite. More generally, we prove a similar result for constructing a coherent subsheaf of a coherent sheaf by prescribing its stalks at all points in a manner consistent with specializations of points, again provided the set of all associated points of all the corresponding local quotients is locally finite. On any l…
An Alternative Model for Coherent Sheaves over Noetherian Schemes
本文利用诺特概形点集的特化偏序构造凝聚层范畴的等价模型，通过建立结构层等价概念和全忠实函子，给出凝聚层范畴的偏序集语言描述。
Triangular spectra and their applications to derived categories of noetherian schemes
本文引入三角谱，从诺特概形的完美导出范畴重构该概形，证明特殊情形下的 Bondal-Orlov-Ballard 重构定理，并比较三角谱与 Balmer 谱。
Let $A$ be a local noetherian ring and $N$ be a locally sheaf on the projective space $P^3_A$ : one proves easily that there exists a family $C$ of (smooth connected) curves contained in $P^3_A$, flat over $A$, and an integer $h$ such that the ideal sheaf $J$ of $C$ has a resolution $0\to P\to N\to J\to 0$ where $P$ is a direct sum of invertible sheaves $O_P(-n_i)$. In this paper we determine, for a given sheaf $N$, all the families of curves with such a resolution, especially the minimal ones (corresponding to the minimum value of $h$). It gives a description of the biliaison class related to…
Let π: X -> S be a finite type morphism of noetherian schemes. A smooth formal embedding of X (over S) is a bijective closed immersion X -> \frak{X}, where \frak{X} is a noetherian formal scheme, formally smooth over S. An example of such an embedding is the formal completion \frak{X} = Y_{/X} where X \subset Y is an algebraic embedding. Smooth formal embeddings can be used to calculate algebraic De Rham (co)homology. Our main application is an explicit construction of the Grothendieck residue complex when S is a regular scheme. By definition the residue complex is the Cousin complex of π^{!} …
The classical HKR-theorem gives an isomorphism of the n-th Hochschild cohomology of a smooth algebra and the n-th exterior power of its module of Kähler differentials. Here we generalize it for simplicial, graded and anticommutative objects in ``good pairs of categories''. We apply this generalization to complex spaces and noetherian schemes and deduce two decomposition theorems for their (relative) Hochschild cohomology (special cases of those were recently shown by Buchweitz-Flenner and Yekutieli). The first one shows that Hochschild cohomology contains tangent cohomology: $\HH^n(X/Y,\sM)=\c…
Let $X$ be a smooth algebraic variety over $k$. We prove that any flat quasicoherent sheaf on $\operatorname{Ran}(X)$ canonically acquires a D-module structure. In addition, we prove that, if the geometric fiber $X_{\overline{k}}$ is connected and admits a smooth compactification, then any line bundle on $S \times \operatorname{Ran}(X)$ is pulled back from $S$, for any locally Noetherian $k$-scheme $S$. Both theorems rely on a family of results which state that the (partial) limit of an $n$-excisive functor defined on the category of pointed finite sets is trivial.
The tangent space to the space of 0-cycles
研究在诺特概型\(S\)及\(S\)上概型\(X\)的基础上，构造\({\rm Sym}^{\infty }(X/S)^+\)相关层，证明\(0 -\)循环空间上一点处平展邻域范畴性质，通过茎函子得出切层茎及切空间。
Let $A$ be a sheaf of Azumaya algebras over a Noetherian base $S$. In this paper we describe using generalized Severi-Brauer varieties, a quasi-projective moduli space parametrizing sheaves of ètale subalgebras of $A$. In the case that $S$ is the spectrum of a field, we study the geometry of this moduli space, and show that in certain cases it is a rational variety.
Let X be a projective variety, $σ$ an automorphism of X, L a $σ$-ample invertible sheaf on X, and Z a closed subscheme of X. Inside the twisted homogeneous coordinate ring $B = B(X, L, σ)$, let I be the right ideal of sections vanishing at Z. We study the subring R = k + I of B. Under mild conditions on Z and $σ$, R is the idealizer of I in B: the maximal subring of B in which I is a two-sided ideal. We give geometric conditions on Z and $σ$ that determine the algebraic properties of R, and show that if Z and $σ$ are sufficiently general, in a sense we make precise, then R is left and right no…
We prove a sheaf-theoretic derived-category generalization of Greenlees-May duality (a far-reaching generalization of Grothendieck's local duality theorem): for a quasi-compact separated scheme X and a "proregular" subscheme Z---for example, any separated noetherian scheme and any closed subscheme---there is a sort of sheafified adjointness between local cohomology supported in Z and left-derived completion along Z. In particular, the i-th left-derived completion functor is the "local homology" sheaf $Ext^i(\RΓ_ZØ_X, -)$. Sheafified generalizations of a number of duality theorems scattered abo…
We consider dominant, generically algebraic, and tamely ramified (if the characteristic is positive) morphisms $π: X/S \to Y/S$, where Y,S are Noetherian and integral and X is a Krull scheme (e.g. normal Noetherian), and study the sheaf of tangent vector fields on Y that lift to tangent vector fields on X. We give an easily computable description of these vector fields using valuations along the critical locus. We generalise to positive characteristic Seidenberg's theorem on lifting tangent vector fields to the normalisation. If $π$ is a blow-up of a coherent ideal I we show that tangent vecto…
Let $π:\CA\ra S$ be an abelian scheme over a scheme $S$ which is quasi-projective over an affine noetherian scheme and let $\CL$ be a symmetric, rigidified, relatively ample line bundle on $\CA$. We show that there is an isomorphism \det(π_*\CL)^{øtimes 24}\simeq\big(π_*ω_{\CA}^{\vee}\big)^{øtimes 12d} of line bundles on $S$, where $d$ is the rank of the (locally free) sheaf $π_*\CL$. We also show that the numbers 24 and $12d$ are sharp in the following sense: if $N>1$ is a common divisor of 12 and 24, then there are data as above such that \det(π_*\CL)^{øtimes (24/N)}\not\simeq\big(π_*ω_{\CA}…
Higher Direct Images of the Structure Sheaf Over a Dedekind Domain
本文研究了Dedekind域上结构层的高阶像，证明了proper birational morphisms之间结构层的高阶像的消失，并扩展了Chatzistamatiou--Rülling在正特征完美域上的结果。
Let S be a locally noetherian scheme and consider two extensions G_1 and G_2 of abelian S-schemes by S-tori. In this note we prove that the fppf-sheaf Corr _S(G_1,G_2) of divisorial correspondences between G_1 and G_2 is representable. Moreover, using divisorial correspondences, we show that line bundles on an extension G of an abelian scheme by a torus define group homomorphisms between G and Pic_{ G/S}.
Picard group action on the category of twisted sheaves
本文研究了概形上扭子层的范畴，证明了张量函子自然同构于恒等函子当且仅当该层同构于结构层，并得出Picard群在导出范畴上作用忠实的推论。
Let $X/k$ be a noetherian scheme over a field $k$ of characteristic 0, such that the residue field at its closed points are algebraic extensions of $k$. Let ${\mathfrak g}_{X/k}\subset T_{X/k}$ be an ${\mathcal O}_{X}$-submodule of the tangent sheaf which is closed under the Lie bracket. We construct a stratification of the subset of closed points in $X$ by locally closed subsets that are preserved by ${\mathfrak g}_{X/k}$, on which ${\mathfrak g}_{X/k}$ acts transitively.
Let $S$ be a noetherian scheme and $f\colon X\to S$ be a smooth morphism of relative dimension 1. For a locally constant sheaf on the complement of a divisor in $X$ at over $S$, Deligne and Laumon proved that the universal local acyclicity is equivalent to the local constancy of Swan conductors. In this article, assuming the universal local acyclicity, we show an analogous result of the continuity of local epsilon factors. We also give a generalization of this result to a family of isolated singularities.
Idempotentization of Affine Schemes and Sheaves
本文引入幂等化过程，将仿射概形及其上的Zariski层关联至幂等半环结构，并证明其独立于覆盖选择，在Noetherian情形下可恢复全局截面，同时揭示了交换半环理想结构的拓扑收缩性质。
Linked sheaves of modules
本文引入连通Noetherian概形上模层的链结概念，通过转置和合冲函子定义稳定层的链结，证明其局部性质及在仿射概形上等价于全局截面模的链结，并给出限制、粘合及非整环情形下的存在性判据，在射影情形比较了链结层的不变量。
On the algebraizability of formal deformations in $K$-cohomology
本文证明了光滑本征簇上两个K-上同调函子及其étale层化的可代数化性是稳定双有理不变量，并为此引入了相对K-同调的新概念。
Higher genus reduced Gromov--Witten invariants via desingularizations of sheaves
本文通过 sheaves 的 desingularizations 构造了 stacks，用于定义大规模 GIT 商在所有 genus 中的减少 Gromov-Witten 不变量。
As a formulation of 'codimension-two arguments' in invariant theory, we define a (rational) almost principal bundle. It is a principal bundle off closed subsets of codimension two or more. We discuss the behavior of the category of reflexive modules over locally Krull schemes, the category of the coherent sheaves which satisfy Serre's condition $(S'_2)$ over Noetherian $(S_2)$ schemes with dualizing complexes, the class group, the canonical module, the Frobenius pushforwards, and global $F$-regularity, of a rational almost principal bundle. We give examples of finite group schemes, multisectio…
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