We consider quadric surface fibrations over curves, defined over algebraically closed and finite fields. Our goal is to understand, in geometric terms, spaces of sections for such fibrations. We analyze varieties of maximal isotropic subspaces in the fibers as P^1-bundles over the discriminant double cover. When the P^1-bundle is suitably stable, we deduce effective estimates for the heights of sections over finite fields satisfying various approximation conditions. We also discuss the behavior of the spaces of sections as the base of the fibration acquires singularities.
In this paper we deal with compactifications of affine homology $3$-cells into quadric fibrations such that the boundary divisors contain fibers. We show that all such affine homology $3$-cells are isomorphic to the affine $3$-space $\mathbb{A}^{3}$. Moreover, we show that all such compactifications can be connected by explicit elementary links preserving $\mathbb{A}^{3}$ to the projective $3$-space $\mathbb{P}^{3}$.
Let Q -> B be a quadric fibration and T -> B a family of sextic du Val del Pezzo surfaces. Making use of the recent theory of noncommutative mixed motives, we establish a precise relation between the Schur-finiteness conjecture for Q, resp. for T, and the Schur-finiteness conjecture for B. As an application, we prove the Schur-finiteness conjecture for Q, resp. for T, when B is low-dimensional. Along the way, we obtain a proof of the Schur-finiteness conjecture for smooth complete intersections of two or three quadric hypersurfaces. Finally, we prove similar results for the Bass-finiteness con…
Using deformation theory of rational curves, we prove a conjecture of Sommese on the extendability of morphisms from ample subvarieties when the morphism is a smooth (or mildly singular) fibration with rationally connected fibers. We apply this result in the context of Fano fibrations and prove a classification theorem for projective bundle and quadric fibration structures on ample subvarieties.
Strong approximation for the total space of certain quadric fibrations
本文研究了四元（x，y，z，t）方程q（x，y，z）=P（t），其中q（x，y，z）是整数上的不定三元二次型，P（t）是整系数的单元一元多项式.如果P（t）不是一个常数和多项式的平方的乘积，则强逼近对积分解（x，y，z，t）成立。在一般情况下，我们证明了积分Brauer-Manin条件是强逼近的唯一障碍。我们实际上研究的是任意数域上的类似情形。- Nous étudions les equations à quantified variables（x，y，z，t）à coefficients entiers du type q（x，y，z）=P（t），其中q（x，y，z）est une formatratique entière ternaire indéfinie sur les réels，et P（t）un polynôme à coefficients entiers en une variable. Lorsque le polynôme n'est pas le produit d'une constante et d'un carré de polynôme，nous établissons l'approximation forte for les solutions des equations en entiers（x，y，z，t）.在一般情况下，我们认为，布劳尔…
This is a report of the author's talk at Algebra Symposium 2019 held at Tohoku University. We discuss some improvements of Mori's Bend and Break for sections of del Pezzo fibrations over $\mathbb P^1$.
We develop a strategy to classify the components of the space of sections of a del Pezzo fibration over $\mathbb{P}^{1}$. In particular, we prove the Movable Bend and Break lemma for del Pezzo fibrations. Our approach is motivated by Geometric Manin's Conjecture and proves upper bounds on the associated counting function. We also give applications to enumerativity of Gromov-Witten invariants and to the study of the Abel-Jacobi map.
Let p: M -> B be a Lagrangian fibration on a hyperkahler manifold of maximal holonomy (also known as IHS), and H the generator of the Picard group of B. We prove that the pullback $p^*(H)$ is a primitive class on M.
Let $X$ be a del Pezzo surface over the function field of a complex curve. We study the behavior of rational points on $X$ leading to bounds on the counting function in Geometric Manin's Conjecture. A key tool is the Movable Bend and Break Lemma which yields an inductive approach to classifying relatively free sections for a del Pezzo fibration over a curve. Using this lemma we prove Geometric Manin's Conjecture for certain split del Pezzo surfaces of degree $\geq 2$ admitting a birational morphism to $\mathbb P^2$ over the ground field.
The Voevodsky nilpotence conjecture was proved by Bernardara-Marcolli-Tabuada for certain quadric fibrations, intersections of quadrics, linear sections of Grassmannians, linear sections of determinantal varieties, and Moishezon manifolds, and later by Ornaghi-Pertusi for certain cubic fourfolds and Gushel-Mukai fourfolds. In this paper we refine these results by using the algebraic equivalence relation instead of the nilpotence equivalence relation. Along the way, we address also certain cases of Küchle fourfolds, families of Sextic del Pezzo surfaces and families of Fano fourfolds of K3 type…
We show that the residual categories of quadric surface bundles are equivalent to the (twisted) derived categories of some scheme under the following hypotheses. Case 1: The quadric surface bundle has a smooth section. Case 2: The total space of the quadric surface bundle is smooth and the base is a smooth surface. We provide two proofs in Case 1 describing the scheme as the hyperbolic reduction and as a subscheme of the relative Hilbert scheme of lines, respectively. In Case 2, the twisted scheme is obtained by performing birational transformations to the relative Hilbert scheme of lines. Fin…
Consider a fibration \[p:E\to S^{p-1}\] with fiber $F$. We have the following natural question: Under what conditions does this fibration admit a cross-section? Our purpose is to discuss this problem when the fibration $p$ is the Milnor fibration $f_{\mid}:f^{-1}(S^{p-1}_δ)\cap D^{n}_ε\to S^{p-1}_δ$ with Milnor fiber $F_f$ and the Milnor fibration of arrangements $Q:\mathcal{M}(\mathcal{A})\to \mathbb{C}^\ast$ with fiber $F=F(\mathcal{A})$. Furthermore, we use our results to study the tasking planning problem for the Milnor fibration as a work map and we give the tasking algorithms.
Skew-symmetric complex matrices, pure spinors, the twistor space of the conformal $2n$-sphere, and the Fano variety of linear $n$-folds of a non-singular complex quadric hypersurface in $\mathbb{P}^{2n+1}$
We study K3 surfaces over a number field $k$ which are double covers of extremal rational elliptic surfaces. We provide a list of all elliptic fibrations on certain K3 surfaces together with the degree of a field extension over which each genus one fibration is defined and admits a section. We show that the latter depends, in general, on the action of the cover involution on the fibers of the genus 1 fibration.
Moduli spaces and braid monodromy types of bidouble covers of the quadric
二次曲面Q的双复盖$π: S \mapsto Q$由依赖于四个正整数a，b，c，d的连通族参数化.在b=d的特殊情况下，我们称它们为abc-曲面。这样的Galois覆盖$π$允许一个小的扰动产生一个一般的4元组覆盖的Q与分支曲线$\De$，和一个自然的Lefschetz纤维化从一个小扰动的组成$ π$与第一个投影。我们证明了一个更一般的结果，这意味着对应于$\De$的辫子单值分解确定了abc-曲面的三个整数a，b，c。我们引入了一种新的方法，以区分不稳定等价的分解。这一结果与第一和第三作者以前的结果形成鲜明对比，表明对应于相应自然Lefschetz束的映射类群分解对于具有相同a+c，b值的abc-曲面是等价的。这个结果暗示了具有固定值a+c，b的abc-曲面，尽管是复纯的但不是形变等价的，可能不是正则辛同构的。
Even if there are too many elliptic fibrations to investigate and describe on the singular $K3$ surface $Y_{10}$ of discriminant 72 and belonging to the Apéry-Fermi pencil $(Y_k)$, we find on it many interesting properties. For example some of its elliptic fibrations with 3-torsion section induce by 3-isogeny either an elliptic fibration of $Y_2$, the unique $K3$ surface of discriminant 8, or an elliptic fibration of other $K3$ surfaces of discriminant 72.
