Questions tagged [schemes]

The first purpose of schemes theory is the geometrical study of solutions of algebraic systems of equations, not only over the real/complex numbers, but also over integer numbers (and more generally over any commutative ring with 1). It was finalized by Alexandre Grothendieck, during the 1950s and the 1960s.

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Is this blow-up a line bundle over the projective line

Let $R$ be the ring $\mathbb{C}[a,b,c,d]/(ac-b^2,bd-c^2,ad-bc)$. Let $I$ be the ideal of $R$ generated by $a,d$. Let $X=\operatorname{Spec}(R)$ and $\tilde{X}$ be the blow-up of the affine scheme $X$ ...
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3 votes
1 answer
163 views

Does a smooth relative curve $X/S$ embed into $\mathbb{P}^3_S$?

Suppose that $\pi:X\to S$ is a smooth projective morphism of relative dimension 1. If $S$ is the spectrum of an algebraically closed field, then it is known that $X$ embeds into $\mathbb{P}^3_S$. ...
1 vote
0 answers
103 views

Check whether a closed point of a Noetherian affine scheme is a local complete intersection

Suppose that $k$ is an algebraically closed field and $A$ is the ring $k[a,b,c,d]/(ac-b^2,bd-c^2,ad-bc)$. Let $X$ be Spec$A$, and $m$ be the maximal ideal of $A$ generated by the quotient images of $a,...
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1 vote
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85 views

How to compute the G-theory groups of a blow-up of Noetherian schemes

Suppose that $k$ is an algebraically closed field and $R$ is a finitely generated $k$-algebra such that if $X$ denotes Spec$R$, then the only closed, singular point of $X$ is the origin. Let $\tilde{X}...
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2 votes
1 answer
136 views

Constructibility of the locus of points where the fiber is an isomorphism modulo nilpotents

Let $f: X \rightarrow S$ be a finitely presented morphism of schemes and let $$E = \{s \in S \mid \text{ $X_s$ is a point with residue field $\kappa(s)$ } \}$$ Is $E$ a constructible set? The basic ...
2 votes
0 answers
184 views

Henselisation of normal rings (Milne's EC)

The usual way to define the Henselisation $A^h$ of a local ring $(A, \mathfrak{m})$ is by taking the direct limit $\varinjlim (B, q)$ over all etale neighborhoods of $A$ (i.e. pairs $(B,q)$ where $B$ ...
3 votes
1 answer
244 views

Normal schemes and Serre's criterion

Serre's criterion says that for a scheme to be normal is equivalent to it being $R_1$ (i.e. regular in codimension $1$) and $S_2$ (i.e. regular functions on $X-Y$ extend to $Y$ if $Y$ has codimension ...
0 votes
0 answers
121 views

The genus of hyperplane sections

Let $S$ be a connected smooth projective surface over $\mathbb{C}$. Let $\Sigma$ be the complete linear system of a very ample divisor $D$ on $S$, and let $d=\dim(\Sigma)$ be the dimension of $\...
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2 votes
0 answers
90 views

Control on the locus of bad reduction for divisors

Let $X$ be a smooth variety over a number field $K$ and let $\mathcal X$ be a normal, projective model of $X$ over the ring of integers $O_K$. Now assume that $D\subset X$ is an irreducible divisor ...
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14 votes
3 answers
1k views

Making the étale topos construction a fully faithful 2-functor from schemes to Grothendieck topoi

For nice topological spaces (say Haudorff spaces) $X$ and $Y$, there is a bijection between continuous maps $X\to Y$ and isomorphism classes of geometric morphisms $\mathrm{Sh}(X)\to \mathrm{Sh}(Y)$. ...
4 votes
0 answers
124 views

Uniqueness of Galois descent

Let $Y,Z$ be $\mathbb{F}_p$-schemes, they are both models of scheme $X$ over $\overline{\mathbb{F}_p}$ . Let $F$ be the absolute Frobenius of $\overline{\mathbb{F}_p}$, If $1_Y\times F$ and $1_Z\times ...
1 vote
0 answers
86 views

Intersection multiplicity in flat families of linear spaces

Let $X\subset\mathbb{P}^N$ be an irreducible projective variety and $\{H_t\}_{t\in \mathbb{C}^{*}}$ a family of $(k-2)$-dimensional linear subspaces of $\mathbb{P}^N$ intersecting $X$ in $k$ distinct ...
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10 votes
1 answer
231 views

Finite coverings by closed subschemes

Let $X$ be a scheme. Assume we have two closed subschemes $Y_1$, $Y_2$ that cover $X$ set-theoretically. Are there closed subschemes $Y'_1$, $Y'_2$ with the same underlying sets, such that the ...
6 votes
0 answers
178 views

Does a universal homeomorphism descend ampleness?

All schemes are quasicompact and separated. If $\pi:X\to Y$ is an affine morphism and $L$ is an ample invertible $\mathscr{O}_Y$-module, then $\pi^*L$ is ample on $X$; we say that $\pi$ descends ...
0 votes
0 answers
88 views

Definition of maximal order on an integral scheme

In the paper: The minimal model program for orders over surfaces by Daniel Chan and Colin Ingalls, they give the following definition. Let $Z$ be an integral normal scheme with quotient field $K$. An ...
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3 votes
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187 views

Is the "naive" version of Chevalley's theorem still true?

Reposting from math.se in case more people are interested here. Chevalley's theorem says that if $f \colon X \to Y$ is a morphism of finite presentation of schemes and $C \subset X$ is constructible, ...
1 vote
0 answers
151 views

Meaning of "cut out (scheme-theoretically)"

Let $V$ be a projectively normal closed subvariety of some projective space over an algebraically closed field $\mathbb{K}$. Let $R$ be the local ring at the vertex of the affine cone over $V$ ($R$ is ...
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1 vote
1 answer
99 views

Arithmetic ampleness and scalings of the metric

Let $\overline L= (L, h)$ be a hermitian $C^ \infty$ line bundle on an arithmetic variety $X\to\operatorname{Spec }\mathbb Z$ (I am reasoning in terms of higher Arakelov geometry, like in Gillet & ...
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2 votes
0 answers
149 views

Is the homotopy limit of derived schemes along affine maps a derived scheme?

The title question is true in the setting of ordinary limits and ordinary schemes; that is, given an inverse limit of schemes along affine maps, the limit still lives in the category of schemes. I'd ...
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2 votes
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185 views

Terminology in log geometry

A log scheme consists of a scheme $X$, a sheaf of monoids $M_X$ on $X$, and a map $\alpha:M_X\to\mathcal O_X$ with the property that $\alpha^{-1}(\mathcal O_X^\times)\to\mathcal O_X^\times$ is an ...
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3 votes
0 answers
140 views

For a family of short exact sequences of coherent sheaves, can we define the splitting subscheme?

This question has been asked in SE. Let $k$ be an algebraically closed field of characteristic zero. Let $X$ be a projective scheme over $k$. We can talk about short exact sequences of coherent ...
0 votes
0 answers
68 views

General fiber and the symmetric product of an ample hypersurface

Let $Sym^m(X)$ be the $m$th symmetric product of a smooth projective variety $X$, $n=\dim(X)$, $Y_1$ an ample hypersurface of $X$, and $CH_0(X)_{hom}$ the Chow groups of $0$-cycles of degree $0$....
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1 vote
1 answer
156 views

Meaning of torsion points in a Roitman's theorem

I am having some problems to understand the meaning of the following theorem due to Roitmann. I found this theorem in Voisin's book: Hodge Theory and Complex Algebraic Geometry, Volume II, page ...
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2 votes
0 answers
93 views

About finite dimensionality of Chow groups of zero cycles

Let $S$ be a connected smooth complex projective surface. Let $Sym^{d}(S)$, $d\in \mathbb{Z}^+_0$, be the $d$-th symmetric product of $S$ parametrizing $0$-cycles of degree $d$. Let $Sym^{d,d}(S)=...
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8 votes
1 answer
432 views

What are the rational functions on a noetherian affine scheme?

Let $A$ be a noetherian ring and $X=\operatorname {Spec}A$ the corresponding affine scheme. There are three rings which might reasonably be called the ring of rational functions on $X$. a) The total ...
3 votes
1 answer
249 views

When Hom scheme has projective components?

The Hom scheme of two projective varieties over some field is constructed as an open subfunctor of the Hilbert scheme of the product of the two schemes by Grothendieck. So it is a countable union of ...
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2 votes
1 answer
106 views

Sheaves on families of genus 2 curves in Hassett's paper

Sorry for a maybe stupid long question but I'm reading the paper "Classical and minimal models of the moduli space of curves of genus two" by Brendan Hassett and I'm not able to unravel a ...
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3 votes
0 answers
110 views

Is the cohomology $H^1(X, \mathcal{E}^\nabla)$ trivial, for the sheaf of constants of an algebraic connection $\nabla$?

Suppose that $\pi:X\to S$ is a flat morphism between Noetherian, integral schemes (of characteristic zero, if need be). Let $\mathcal{E}$ be a locally free sheaf on $X$, and $$\nabla:\mathcal{E} \to \...
2 votes
0 answers
48 views

Conditions for long exact sequence for line bundles on curve to degenerate?

Let $\varphi:X\to Y$ be a morphism of schemes of relative dimension 1, and $\mathcal{L}' \xrightarrow{g} \mathcal{L}$ an injection of line bundles on $X$. The sequence $$0\to \mathcal{L}' \xrightarrow{...
5 votes
2 answers
311 views

Connectedness of Quot schemes

Let $X$ be a connected projective scheme over $\mathbb{C}$ and $E$ a coherent sheaf on $X$. Consider the Quot scheme $\operatorname{Quot}_X(E,P)$ of quotients of $E$ of fixed Hilbert polynomial $P$. ...
1 vote
0 answers
239 views

On construction of Hilbert and Quot schemes

I have some questions regarding the strategy of the proof of the existence of Hilbert and Quot schemes (I will focus on the latter since it's more general), as in the book Fundamental Algebraic ...
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3 votes
1 answer
256 views

Schemes as categories fibered in thin groupoids

Every time I start to read about schemes from a birds-eye view (like in the introduction to The Geometry of Schemes by Eisenbud and Harris) I get really excited; they sound like a categorical approach ...
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2 votes
0 answers
245 views

What algebraic condition corresponds to injectivity of a morphism of varieties?

$\DeclareMathOperator{\Spec}{Spec}$ Let $X = \Spec A$, $Y = \Spec B$ be affine complex varieties, that is reduced $\mathbb{C}$-schemes of finite type. Equivalently we can say that $A$ and $B$ are ...
5 votes
1 answer
319 views

Proper morphisms that are closed immersion on a fiber

I am interested in a morphism of $S$-schemes $f : X \to Y$ such that $X$ and $Y$ are proper over $S$ and there is some $s_0 \in S$ such that $f : X_{s_0} \to Y_{s_0}$ is a closed immersion. Is it true ...
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3 votes
1 answer
214 views

Galois invariant line bundle and base change

Let $K$ be a number field and consider a finite Galois extension $L|K$. Moreover let $X$ be a projective, regular, integral variety over $K$. After a base change we obtain a morphism of varieties $f:...
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1 vote
0 answers
159 views

Zeta functions of schemes of finite type over $\mathbb{Z}$

Let $X$ be a scheme of finite type over $\mathbb{Z}$. In Section 11 of my 2008 paper in J. Number Theory, "Ring structures on groups of arithmetic functions," I define an additive analogue $...
3 votes
0 answers
193 views

Ind-etale vs weakly etale

In this article Bhatt and Scholze consider ind-etale and weakly etale maps of affine schemes. We have two (easy) statements, proven in Prop.2.3.3(1) and (5): -- any ind-etale map is weakly etale, -- ...
2 votes
1 answer
316 views

Why geometric generic point (in abstract algebraic geometry) replace general points in the unit disk?

In section 4.1, chapter 4 of Pierre Deligne's paper La conjecture de Weil : I (french version, translation to English) he states: On $\mathbb{C}$ Lefshietz local results are as follows. Let $X$ be a ...
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3 votes
2 answers
377 views

Finite etale cover of projective line

If we have a map $f : \mathbb P^1_R \to \mathbb P^1_R$ over $\operatorname{Spec}(R)$, with $R$ a commutative ring, which we assume to be etale, then is it possible to characterize $f$? Must it be an ...
2 votes
1 answer
155 views

Epi-mono factorisations in schemes via scheme-theoretic image

Suppose that $f : X \rightarrow Y$ is a morphism of schemes. Let $Z \hookrightarrow Y$ the scheme-thereotic image of $f$. Under what conditions is the morphism $X \rightarrow Z$ an epimorphism? If ...
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1 vote
1 answer
174 views

Are there nonaffine schemes over which every exact sequence of vector bundles is split?

Is there an example of a non-affine scheme $X$ such that every short exact sequence of vector bundles over $X$ splits? If there are such examples then what if we ask it to be true of all (not ...
1 vote
0 answers
69 views

The weight of a weighted filtration is given (for large $m$) by a polynomial

Let $I$ be an homogeneous ideal of $k[x_0, \dots, x_n]$. Suppose to give integral weights $\lambda_0, \dots, \lambda_n$ to $x_0, \dots, x_n$. We assign a weight to every homogeneous polynomial of ...
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3 votes
0 answers
80 views

Do rationally contractible presheaves have rationally contractible injective resolution

Given a presheaf $\mathcal{F}: Sm/k\rightarrow Ab$ we define a new presheaf $C\mathcal{F}= \varinjlim\limits_{X\times \{0,1\}\subset U \subset X\times \mathbb{A}^1}\mathcal{F}(U)$. The presheaf $\...
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0 votes
0 answers
88 views

Closed algebraic subset dominating a curve

In the book "C. Voisin. Hodge Theory and Complex Algebraic Geometry. Volume II. Cambridge studies in advanced mathematics 76 (2002)" page 228 says: Let $X$ be a smooth projective variety. If ...
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1 vote
0 answers
118 views

integral subschems of geometrically integeral schemes

Let $X$ be a geometrically integral surface over a field $k$ and let $C$ be an integral curve on $X$. I want to know whether $C$ is geometrically irreducible over $k$ or not.
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1 vote
0 answers
112 views

Help to use Statistics and algebra books for community [closed]

My father has 2000 statistics and higher algebra books (schaum series etc). Need to use these for community since he passed away (India) kindly guide me I just need to know if we can donate these ...
2 votes
1 answer
380 views

Extending functors between K-algebras to schemes

Assume we have $K$ and $L$ (comm.) rings, and we have a functor $F$ from the category of $K$-Algebras to the category of $L$-Algebras (I work only with commutative rings). What conditions need to ...
1 vote
1 answer
368 views

Intuition behind formal neighborhood and local ring and formal power series

In The Geometry of Schemes by David Eisenbud and Joe Harris, on page 57, there is an explanation on "node" of a plane curve. The book says that, a curve $X\subseteq \mathbb A_{\mathbb C}^2$ ...
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0 votes
0 answers
231 views

Flatness of affine cone due to semicontinuity theorem

I would like to clarify an important aspect from the discussion in this question. The OP discussed an obstacle to solve part (c) from Exercise 9.5 from Hartshorne's Algebraic Geometry Chap. III page ...
1 vote
0 answers
112 views

Combinatorics of projective planes over commutative rings

An axiomatic projective plane is a point-line incidence structure with the following axioms: any two distinct points are collinear (via a unique line); any two distinct lines meet in a unique point; ...
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