Ichiro Shimada
We present a method to produce examples of non-homeomorphic conjugate complex varieties based on the genus theory of lattices. As an application, we give examples of arithmetic Zariski pairs.
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We show that every affine or projective algebraic variety defined over the field of real or complex numbers is homeomorphic to a variety defined over the field of algebraic numbers. We construct such a homeomorphism by choosing a small deformation of the coefficients of the original equations. This method is based on the properties of Zariski equisingular families of varieties. Moreover we construct an algorithm, that, given a system of equations defining a variety $V$, produces a system of equations with algebraic coefficients of a variety homeomorphic to $V$
We give examples of smooth quasi-projective varieties over complex numbers, in the context of connected Shimura varieties, which are not homeomorphic to a conjugate of itself by an automorphism of the complex numbers.
Topology of Galois conjugate character varieties
研究特征簇上同调相关结构的相互作用，提出检测伽罗瓦共轭特征簇同伦类型差异的方法，给出第一对非同伦等价的伽罗瓦共轭特征簇例子，否定豪塞尔2005年的问题。
For any subfield K of the complex numbers which is not contained in an imaginary quadratic number field, we construct conjugate varieties whose algebras of K-rational (p,p)-classes are not isomorphic. This compares to the Hodge conjecture which predicts isomorphisms when K is contained in an imaginary quadratic number field; additionally, it shows that the complex Hodge structure on the complex cohomology algebra is not invariant under the Aut(\C)-action on varieties. In our proofs, we find simply connected conjugate varieties whose multilinear intersection forms on their second real cohomolog…
Using constructions of Voisin, we exhibit a smooth projective variety defined over a number field k and two complex embeddings of k, such that the two complex manifolds induced by these embeddings have non isomorphic cohomology algebras with real coefficients. This contrasts with the fact that the cohomology algebras with l-adic coefficients are canonically isomorphic for any prime number l, and answers a question of Grothendieck.
Serre and Abelson have produced examples of non-homeomorphic conjugate varieties. We show that if the field of definition of a polarized projective variety coincides with its field of moduli then all of its conjugates have the same topological type. This extends the class of varieties known to posses conjugacy invariant to canonically embedded varieties. We also show that normal complete interswections in homogeneous varieties have this property.
We show that conjugation by an automorphism of the complex numbers (as an abstract field) may change the topological fundamental group of a locally symmetric variety over C. As a consequence, we obtain a large class of algebraic varieties defined over number fields with the property that different embeddings of the number field into C give complex varieties with nonisomorphic fundamental groups.
Miyaoka-Yau inequalities and the topological characterization of certain klt varieties
本文研究具有klt奇点的射影簇的拓扑刻画问题，将复射影流形上关于球商空间、阿贝尔簇及射影空间的拓扑刻画与陈类刻画相关结果，推广到带klt奇点的对应奇异簇类中。
Quivers play an important role in the representation theory of algebras, with a key ingredient being the path algebra and the preprojective algebra. Quiver grassmannians are varieties of submodules of a fixed module of the path or preprojective algebra. In the current paper, we study these objects in detail. We show that the quiver grassmannians corresponding to submodules of certain injective modules are homeomorphic to the lagrangian quiver varieties of Nakajima which have been well studied in the context of geometric representation theory. We then refine this result by finding quiver grassm…
Let $S$ be a Shimura variety with reflex field $E$. We prove that the action of $\operatorname{Gal}(\overline{\mathbb{Q}}/E)$ on $S$ maps special points to special points and special subvarieties to special subvarieties. Furthermore, the Galois conjugates of a special point all have the same complexity (as defined in the theory of unlikely intersections). These results follow from Milne and Shih's construction of canonical models of Shimura varieties, based on a conjecture of Langlands which was proved by Borovoi and Milne.
Rigid analytic 1-motives and conjugate uniformization of abeloid varieties
本文针对p进域上的阿贝尔簇，研究刚性解析1-动机理论及阿贝尔簇的共轭单值化，建立了形式与刚性解析1-动机的等价性等结果，还构造了相关p可除群。
Non-cancellative varieties, maximal tori, and the Makar-Limanov invariant
研究广义扎里斯基消去问题，构建新反例，通过反例上柱体的极大环面及非刚性三项式簇上柱体的性质研究，证明相关结论并得出其马卡洛夫 - 利马诺夫不变量平凡的结果。
We study the Lipschitz Geometry of Complex Algebraic Singularities. For this purpose we introduce the notion of choking horns. A Choking horn is a family of cycles on the family of the sections of an algebraic variety by very small spheres centered at a singular point, such that the cycles cannot be boundaries of nearby chains. The presence of choking horns is an obstruction to metric conicalness as we can see with some classical isolated hypersurfaces singularities which we prove are not metrically conic. We also show that there exist infinitely countably many singular varieties, which are lo…
We obtain a non--trivial upper bound for the multiplicative energy of any sufficiently large subset of a subvariety of a finite algebraic group. We also find some applications of our results to growth of conjugates classes, estimates of exponential sums and the restriction phenomenon.
We develop a sheaf cohomology theory of algebraic varieties over an algebraically closed non-trivially valued non-archimedean field $K$ based on Hrushovski-Loeser's stable completion. In parallel, we develop a sheaf cohomology of definable subsets in o-minimal expansions of the tropical semi-group $Γ_\infty$, where $Γ$ denotes the value group of $K$. For quasi-projective varieties, both cohomologies are strongly related by a deformation retraction of the stable completion homeomorphic to a definable subset of $Γ_\infty$. In both contexts, we show that the corresponding cohomology theory satisf…
Inspired by a construction by Arnaud Beauville of a surface of general type with $K^2 = 8, p_g =0$, the second author defined the Beauville surfaces as the surfaces which are rigid, i.e., they have no nontrivial deformation, and admit un unramified covering which is isomorphic to a product of curves of genus at least 2. In this case the moduli space of surfaces homeomorphic to the given surface consists either of a unique real point, or of a pair of complex conjugate points corresponding to complex conjugate surfaces. It may also happen that a Beauville surface is biholomorphic to its complex …
Fence Complexes and Toric Degenerations of Positroid Varieties
本文研究了正体种集的围栏复形结构，证明其与闭球同胚，并展示其在正体种集的非负部分上提供正则CW复形结构，同时证明围栏复形的Ehrhart多项式等于相关正体种集的Hilbert多项式。
In a previous work, the third named author found a combinatorics of line arrangements whose realizations live in the cyclotomic group of the fifth roots of unity and such that their non-complex-conjugate embedding are not topologically equivalent in the sense that they are not embedded in the same way in the complex projective plane. That work does not imply that the complements of the arrangements are not homeomorphic. In this work we prove that the fundamental groups of the complements are not isomorphic. It provides the first example of a pair of Galois-conjugate plane curves such that the …
In a seminal 1994 paper, Lusztig extended the theory of total positivity by introducing the totally non-negative part (G/P)_{\geq 0} of an arbitrary (generalized, partial) flag variety G/P. He referred to this space as a "remarkable polyhedral subspace", and conjectured a decomposition into cells, which was subsequently proven by the first author. Subsequently the second author made the concrete conjecture that this cell decomposed space is the next best thing to a polyhedron, by conjecturing it to be a regular CW complex that is homeomorphic to a closed ball. In this article we use discrete M…
Real phase structures on tropical manifolds and patchworks in higher codimension
本文用热带几何将Viro组合补丁构造推广到任意余维，证明热带非奇异补丁为拓扑流形，并推广谱序列得到贝蒂数上界。
Hyperbolicity and fundamental groups of complex quasi-projective varieties (II): via non-abelian Hodge theories
本文通过非阿贝尔霍奇理论研究复准项目ive簇的双曲性，证明了在特定表示下，非双曲性位置的性质，并扩展了前人关于项目ive簇的结果。
This paper gives a conjectural characterization of those elliptic curves over the field of complex numbers which "should" be covered by standard modular curves. The elliptic curves in question all have algebraic j-invariant, so they can be viewed as curves over Q-bar, the field of algebraic numbers. The condition that they satisfy is that they must be isogenous to all their Galois conjugates. Borrowing a term from B.H. Gross, "Arithmetic on elliptic curves with complex multiplication," we say that the elliptic curves in question are "Q-curves." Since all complex multiplication elliptic curves …
An Ogus Principle for Zip period maps: The Hasse invariant's vanishing order via `Frobenius and the Hodge filtration'
本文将 Ogus 关于 Calabi–Yau 族 Hasse 不变量消失阶与共轭线位置的结果推广到群论 Zip 周期映射，并导出 Shimura 簇及 K3 模空间上的原理。
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