Algebraic varieties, stacks, sheaves, schemes, moduli spaces, complex geometry, quantum cohomology.

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2 views

isogeny clases of CM abelian varieties

Let $A$ be an abelian variety defined over $\overline{\mathbb{Q}}$ and with complex multiplication by a CM field $K$. Looking at the action of $K$ on $H^0(A, \Omega^1_A)$ one gets a CM type of $K$, ...
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1answer
88 views

Smooth morphism to homogeneous spaces and fibers

Let $f:X \to Y$ be a smooth morphism between projective varieties. Suppose $Y$ is a homogeneous space. Under what additional condition on $f$, can we conclude that every fibers of $f$ are isomorphic?
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72 views

Artin's criterion for étale, quasi-separated algebraic spaces

it is known from Knutson's work that an algebraic space which is separated and étale over a scheme is a scheme. Let $S$ be a locally noetherian scheme. I am looking for a reference giving an Artin's ...
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1answer
80 views

Picard group of a quotient of a group by its maximal parabolic subgroup

Let $G$ be a connected, linear, semi-simple algebraic group over an algebraically closed field of characteristic zero and $P$ be the maximal parabolic subgroup. We know that the quotient $Z=G/P$ is a ...
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0answers
53 views

Functor of order $n$ in Mumford's abelian variety

Let $T$ be a contravariant functor on the category of complete varieties into the Category $\underline{\mathrm{Ab}}$ of abelian groups. Let $X_0,\ldots,X_n$ be any system of complete varieties, ...
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74 views

Independent Generic Curves in the Projective Plane

I'm trying to read M. Nagata's paper On the Fourteenth Problem of Hilbert and I've run into some trouble understanding the definitions he's using. The setup is as follows: let $\pi$ be a prime field ...
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140 views

Different proof's of Marten's theorem

I am referring to Marten's theorem on the dimension of $W_d^r $ as in ACGH p. 192 . It seems to me that an even shorter proof can be given using Hopf's Theorem that if $\nu : A \otimes B \to C $ is ...
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162 views

What is deforming this non-complete intersection like?

Let $R = \mathbf{C}[x,y,u,v]$ be the coordinate ring of $\mathbf{C}^4$. Let $I$ be the ideal generated by $u$ and $v$, let $J$ be the ideal generated by $u$ and $y$. What are the flat deformations of ...
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87 views

Meromorphic functions on $U^2 = T^3 + 1$, genus [on hold]

This question Asked in S.E but no, answer ,I would like to know how do i find a genus of $F$ . Let $k$ be a field of characteristic $\neq 2$, and consider the quadratic extension $F$ of $k(T)$ ...
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0answers
161 views

Can estimate upper bound of $|p_{i}|$ or $|q_{i}|?$

when I Find the diophantine-equation rational points $$2y^2=x^6-x^2+2$$ I using Faltings's theorem showed that there are only finitely many solutions,if we assmue that ...
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0answers
43 views

Clockwise sorting of circle point [on hold]

I have list of 3d points ( -2.03591339559,-0.560307972035,-0.474112849094), ( -2.05118196203,-0.55785528461,0.5743518821), ( -1.02999710644,1.16145402736,0.585203882893), ( ...
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72 views

A question about Segre class

Suppose $C$ is a cone over $X$.(i.e.$C=\operatorname{Spec}S$, where $S$ is a sheaf of $O_X$ algebras.) The Segre class $s(C)$of $C$ is the class in $A_*(X)$ defined by ...
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71 views

How can I keep the roots of f(x)^n+g(x)^m far away from the roots of f and g?

More specifically, suppose for example I have $h(x)=\sum_{i=1}^k (x-i)^{d_i}$. Can I get any handle on the roots of $h(x)$? Can I somehow guarantee that the roots of $h(x)$ are not arbitrarily close ...
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1answer
162 views

Decidability of an Algebraic System in Real Numbers

Is there an algorithm to decide whether an algebraic system \begin{gathered} {f_1}({x_1}, \ldots ,{x_n}) = 0 \hfill \\ \vdots \hfill \\ {f_m}({x_1}, \ldots ,{x_n}) = 0 \hfill \\ ...
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0answers
108 views

What is known about order of torsion of jacobian of hyperelliptic curve over finite field? [on hold]

Suppose $J$ is jacobian of hyperelliptic curve $C$ over $F_p$ of genus $g$. Suppose $T$ is torsion of $J(F_p)$. What is known about order of $T$? Are there some bounds on order of $T$? Can one say ...
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96 views

field of constants of a curve [on hold]

I'm trying to gain some intuition about the field of constants of a curve. If $C$ is over a field $k$, then it is defined as the set of elements of $k(C)$ algebraic over $k$. If I understood ...
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0answers
124 views

Conditions for splitting of short exact sequence?

Are there conditions under which the short exact sequence $$0\rightarrow E (K)/mE (K)\rightarrow H^1_{Sel}(K,E_m)\rightarrow \Sha(E|K)_m\rightarrow 0$$ splits? I assume $K $ to be a number field and ...
14
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4answers
650 views

Number of $\mathbb F_p$ points constant mod $p$?

I have some affine varieties $X$ defined over $\mathbb Z$, and associated integers $c(X)$, with the property that $\# X_{\mathbb Z/p} \equiv c(X) \bmod p$ for all $p$. (In particular $c(X)$ is usually ...
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0answers
87 views

References for modular curves over finite fields [on hold]

I'm looking for a detailed reference for modular curves over finite fields, such as $X(N)$, $X_1(N)$, and $X_0(N)$. There seems to be a lot of literature dealing with them over $\mathbb{C}$, but I'm ...
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1answer
100 views

rationality of residues of differentials

Let $C$ be a smooth curve over a field $k$, $\overline{C}$ the smooth compactification and $S=\overline{C} \setminus C$. We think of $S$ as a reduced divisor defined over $k$. Take the sheaf of ...
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2answers
210 views

Theta characteristics of genus$\geq3$ curve

Let $C$ be a smooth curve of genus$\geq3$ over $\mathbb{C}$, so there are $2^{g-1}(2^g-1)$ odd theta characteristics and $2^{g-1}(2^g+1)$ even theta characteristics. Do we know how many of them has ...
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2answers
299 views

Specialisation of rigid varieties

Recall that a variety $X$ over a field $k$ is called rigid if $H^1(X, T_X) = 0$. I am interested in understanding this property under specialisation. Let $R$ be a discrete valuation ring and let ...
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1answer
319 views
+200

Deformations of Ext rings

Let $k$ be a base ring and $k[x]$ the ring of polynomials in an indeterminate $x$ over $k$. Consider a (not necessarily commutative) algebra $A$ over $k[x]$ and two $A$-modules $M$ and $N$. Then for ...
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1answer
133 views

Morphisms contracting a family of curves

Let $f:X\rightarrow Y$ be a morphism of normal projective varieties. Let $S\subseteq X$ be a surface admitting a morphism $g:S\rightarrow C$ to a curve $C$ such that any fiber of $g$ is a curve. ...
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79 views

Looking for examples of holomorphic maps to $\mathbb{P}^1$ with certain property

I would like to know any example of nonconstant holomorphic map $f:X\to\mathbb{P}^1$ such that $K_X\cong f^*\mathcal{O}(2n)$ for some positive integer $n$, where $K_X$ is the canonical bundle of $X$. ...
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1answer
119 views

CM abelian varieties over the rationals

Let $K$ be a number field and let $A$ be an abelian variety of dimension $g$ over $K$. Let $L$ be a CM field and suppose that $[L:{\bf Q}]=2g$. Suppose that there exists an embedding ...
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34 views

Calculating the distinguished varieties of intersection product

In Fulton's Intersection theory Example 6.1.2,one considers two divisors on $\mathbf{P}^2$ given by $D_1=A+2B,D_2=2A+B$, where $A,B$ are lines meeting at a point. Let $X=D_1\times ...
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1answer
116 views

Sections of proper, flat morphism

Let $f:X \to Y$ be a proper, flat morphism of projective scheme and $Y$ is an irreducible, non-singular surface. Assume further that there exists a Zariski open subset $U$ of $Y$ whose complement is ...
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48 views

open set of hyperplanes not meeting a family of lines

Let \begin{align} \Omega=\begin{bmatrix} L_1 & \cdots & L_{n-1} \\ M_1 & \cdots & M_{n-1} \end{bmatrix}\end{align} be a matrix of linear forms on $\mathbb{P}^n$, i.e. homogeneous ...
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1answer
176 views

The stack of group algebraic spaces

The fibred category $\mathcal A$ of algebraic spaces over a scheme $S$ is a stack (over the category of affine schemes with the etale topology). This is proved in Laumon and Moret-Bailly's book (see ...
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65 views

Surjectivity locus of a morphism of families of sheaves

Let $X$ and $T$ be schemes and assume we have two coherent sheaves $\mathcal{F}$ and $\mathcal{G}$ on $X\times T$ which are flat over $T$, that is these are families of sheaves parametrized by $T$. ...
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74 views

a question about Nakayama functor [closed]

Assume $A$ a finite dimesional algebra over $k$, we assume $k$ algebraically closed, then can we computer $Hom_k(RHom_A(Hom_k(A,k),A),k)$ which is $N^2(A)$, here N is the derived Nakayama functor. For ...
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0answers
61 views

a closure argument regarding certain linear functionals on polynomials

Let $S_d$ be the vector space of homogeneous polynomials of degree $d$ in two variables $x,y$ over an algebraically closed field $k$. Let $\phi \in S_d^*$ be a linear functional on $S_d$, such that ...
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1answer
166 views

Stacks with a small coarse moduli space

Let $k$ be a field of characteristic zero. Let $X$ be a finite type algebraic stack over $k$ with a coarse (or good) moduli space $M$. Suppose that $M$ is isomorphic to a point, i.e., $M = Spec k$. ...
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1answer
136 views

Torsors and Central Extensions

In the setting of algebraic groups: I understand that a central extension of a group $G$ by an abelian group $A$ is a exact sequence of groups :$0\rightarrow A\rightarrow \tilde{G}\rightarrow ...
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1answer
235 views

Order of vanishing of an integer polynomial at a point

Let $f(x,y)$ be a polynomial with integer coefficients, and let $\alpha=(\alpha_1,\alpha_2)\in \mathbb{C}^2$ be a complex point. I want to show that $f$ cannot vanish at $\alpha$ to high order unless ...
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2answers
144 views

Pseudo-decision procedures for first order arithmetic

I was reading this paper http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.117.2911&rep=rep1&type=pdf in which the author describes an algorithm, based on Groebner basis, ...
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82 views

Correspondence between a tangent vector and a deformation [closed]

This is from the book "Algebraic Geometry- An Introduction" by D. Perrin at page 90, where tangent spaces are introduced. Can someone please add more explanation for the underlined part (red line) at ...
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1answer
229 views

Kummer theory and ramified covers

Let $X \to Y$ be a cyclic cover of algebraic varieties, with $Y$ smooth and $X$ normal, say over the complex numbers. Let $G$ denote the Galois group and let $\chi$ be a character. By Kummer theory ...
23
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2answers
628 views

How slowly can a power of an ideal grow?

For a polynomial ideal $I\subset \mathbb{C}[x_1,x_2]$, let $D(I)$ be the smallest degree of any polynomial in $I$. How slowly can $D(I^n)$ grow as a function of $n$? For example, if $D(I^n)\leq ...
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2answers
182 views

meaning of $k$-rational for closed subschemes

Let $X$ be a variety over a field $k$. I know the definition of $k$-rational point: a closed point $x$ is $k$-rational if its residue field $k(x)$ is equal to $k$ (in general, $k(x)$ is only a finite ...
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1answer
286 views

About normalization

Let $$f\colon X \to Y$$ be a morphism of affine normal algebraic varieties over $\mathbb{C}$. Assume that $f$ is birational and bijective on closed points. Does normality imply that $f$ is an ...
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1answer
298 views

Is “quotient” of projective variety projective?

Suppose $X$ is a projective variety, $f\colon X\to Y$ is a finite surjective morphism onto variety $Y$, must $Y$ be a projective variety?
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1answer
406 views

Which degree does a motivic Galois representation show up in?

Consider a representation $\rho: \operatorname{Gal} (\overline{\mathbb Q} | \mathbb Q ) \to GL_n ( \overline{\mathbb Q}_\ell)$ that is a subrepresentation of $H^i(X, \overline{\mathbb Q}_\ell (j))$ ...
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1answer
164 views

Does this extension of Hodge structures split over $\mathbb{Q}$?

Let $X$ be a smooth projective curve of genus $\geq 1$ over $\mathbb{C}$, $H^\cdot=H^\cdot(X)$, and $K$ be the kernel of cup product $\cup: H^1\otimes H^1\rightarrow H^2$. Consider the extension of ...
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0answers
103 views

Hodge numbers and weight filtration

Let $X$ be a complex smooth projective variety and $D$ a divisor on $X$ with normal crossings. As usual, denote by $D(m)$ the disjoint union of all possible intersections of $m$ irreducible components ...
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1answer
142 views

When is the normal neighbourhood of the boundary of the moduli space of cuvres parametrized by exactly one branch?

Let $X$ be a compact complex manifold and $\beta \in H_2(X, \mathbb{Z}) $ a fixed homology class that is $\textit{decomposable}$. Let $$ \overline{\mathcal{M}}_{0,n}(X, \beta) $$ denote the stable ...
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1answer
231 views

Equivariant Derived Category

Can someone give me a reference for the following or an idea on why it is true? (This is taken from remark 1.5 on page 5 of http://arxiv.org/abs/0810.0794.) Suppose we have an algebraic group $G$ ...
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120 views

Projective bundle formula

Given a projective bundle $\mathbb{P}(E)\to X$, is there any reference for the projective bundle formula for Hodge cohomology ring $\oplus H^p(X,\Omega_{X/k}^q)$?
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116 views

Are fixed points of automorphisms rational?

Fix an integer $d$ and a number field $K$ containing the $d$-th roots of unity. Let $X$ be a variety over $K$ such that $Aut(X / K)$ has an element $\varphi$ of order $d$. Is it true that the fixed ...