Goulwen Fichou
We define a generalised Euler characteristic for arc-symmetric sets endowed with a group action. It coincides with equivariant homology for compact nonsingular sets, but is different in general. We lay emphasis on the particular case of $Z/2\Z$, and give an application to the study of the singularities of Nash function germs via an analog of the motivic zeta function of Denef & Loeser.
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The weak factorization theorem for birational maps is used to prove that for all nonnegative i the ith mod 2 Betti number of compact nonsingular real algebraic varieties has a unique extension to a "virtual Betti number" beta_i defined for all real algebraic varieties, such that if Y is a closed subvariety of X, then beta_i(X) = beta_i(X\Y) + beta_i(Y).
We study the Jordan normal forms of the local and global monodromies over complete intersection subvarieties of $C^n$ by using the theory of motivic Milnor fibers. The results will be explicitly described by the mixed volumes of the faces of Newton polyhedrons.
Let $\mathrm{R}$ be a real closed field. We prove that for any fixed $d$, the equivariant rational cohomology groups of closed symmetric semi-algebraic subsets of $\mathrm{R}^k$ defined by polynomials of degrees bounded by $d$ vanishes in dimensions $d$ and larger. This vanishing result is tight. Using a new geometric approach we also prove an upper bound of $d^{O(d)} s^d k^{\lfloor d/2 \rfloor-1} $ on the equivariant Betti numbers of closed symmetric semi-algebraic subsets of $\mathrm{R}^k$ defined by quantifier-free formulas involving $s$ symmetric polynomials of degrees bounded by $d$, wher…
Let $\mathrm{R}$ be a real closed field. The problem of obtaining tight bounds on the Betti numbers of semi-algebraic subsets of $\mathrm{R}^k$ in terms of the number and degrees of the defining polynomials has been an important problem in real algebraic geometry with the first results due to Ole{\uı}nik and Petrovski{\uı}, Thom and Milnor. These bounds are all exponential in the number of variables $k$. Motivated by several applications in real algebraic geometry, as well as in theoretical computer science, where such bounds have found applications, we consider in this paper the problem of bo…
Let X be a smooth projective variety with the action of the n dimensional torus. The article describes the moduli space of torus equivariant morphisms from stable toric varieties into X as the inverse limit of the GIT quotients of X and their flips when these spaces are enhanced with a naturally associated Deligne-Mumford stack structure. This description is used for constructing a class in the Chow group of the moduli space of dimension dim(X)-n which is invariant to equivariant deformations of X.
We construct Eagon--Northcott cycles on Hurwitz space and compare their classes to Kleiman's multiple point loci. Applying this construction towards the classification of Betti tables of canonical curves, we find that the value of the extremal Betti number records the number of minimal pencils. The result holds under transversality hypotheses equivalent to the virtual cycles having a geometric interpretation. We analyse the case of two minimal pencils, showing that the transversality hypotheses hold generically.
For a $G$-scheme $X$ with a given equivariant perfect obstruction theory, we prove a virtual equivariant Grothendieck-Riemann-Roch formula, this is an extension of a result of Fantechi-Göttsche to the equivariant context. We also prove a virtual non-abelian localization theorem for schemes over $\mathbb{C}$ with proper actions.
Betti numbers of the moduli space of Higgs bundles over a real curve
本文基于Mellit、Fedorov-Soibelman-Soibelman和Schiffmann的动机公式，推导出实曲线上的稳定Higgs丛模空间M_r^d的Z_2-Betti数公式，并证明虚拟的Z_2庞加莱多项式是实数上的动机度量。
Virtual Riemann-Roch Theorems for Almost Perfect Obstruction Theories
本文在几乎完美障碍理论框架下证明虚拟黎曼-罗赫定理，涵盖非等变与等变情形及余截面局部化版本，推广并简化了既有结果，为K-理论不变量提供坚实基础。
We define invariants of the blow-Nash equivalence of real analytic function germs, in a similar way that the motivic zeta functions of Denef & Loeser. As a key ingredient, we extend the virtual Betti numbers, which were known for real algebraic sets, as a generalized Euler characteristics for projective constructible arc-symmetrics sets. Actually we prove more: the virtual Betti numbers are not only algebraic invariant, but also Nash-invariant of arc-symmetric sets. Our zeta functions enable to sketch the blow-Nash equivalence classes of Brieskorn polynomials of two variables.
Boij-Söderberg theory characterizes syzygies of graded modules and sheaves on projective space. This paper continues earlier work with S. Sam, extending the theory to the setting of $GL_k$-equivariant modules and sheaves on Grassmannians. Algebraically, we study modules over a polynomial ring in $k n$ variables, thought of as the entries of a $k \times n$ matrix. We give equivariant analogues of two important features of the ordinary theory: the Herzog-Kühl equations and the pairing between Betti and cohomology tables. As a necessary step, we also extend previous results, concerning the base c…
On a symplectic generalization of a Hirzebruch problem
受Hirzebruch问题启发，本文研究具有Hamilton环面作用且第二Betti数为1的8维闭辛流形，分类了第四Betti数为2时的等变不变量，并证明了更一般条件下的有限性结果。
We characterize the cone of GL-equivariant Betti tables of Cohen-Macaulay modules of codimension 1, up to rational multiple, over the coordinate ring of square matrices. This result serves as the base case for `Boij-Söderberg theory for Grassmannians', with the goal of characterizing the cones of GL_k-equivariant Betti tables of modules over the coordinate ring of k x n matrices, and, dually, cohomology tables of vector bundles on the Grassmannian Gr(k, C^n). The proof uses Hall's Theorem on perfect matchings in bipartite graphs to compute the extremal rays of the cone, and constructs the corr…
(Equivariant) Cohomology of Homogeneous Spaces Associated with Composition Algebras
研究由2维和4维结合合成代数嵌入产生的齐次空间，通过等变微分同胚和形变收缩将上同调计算约化到紧模型，并利用Weyl群不变量理论计算有理等变上同调环和Poincaré多项式。
To any Nash germ invariant under right composition with a linear action of a finite group, we associate its equivariant zeta functions, inspired from motivic zeta functions, using the equivariant virtual Poincaré series as a motivic measure. We show Denef-Loeser formulae for the equivariant zeta functions and prove that they are invariants for equivariant blow-Nash equivalence via equivariant blow-Nash isomorphisms. Equivariant blow-Nash equivalence between invariant Nash germs is defined as a generalization involving equivariant data of the blow-Nash equivalence.
Virtual Localization in equivariant Witt cohomology
本文在 $\ ext{SL}_2$ 中环面正规化子作用且以 Witt 群层上同调替代 Chow 群的设定下，证明了 Graber-Pandharipande 虚拟局部化定理的类比。
Equivariant Segre and Verlinde invariants for Quot schemes
本文研究 C^2 与 C^4 上 Quot 概形虚拟几何中的等变 Segre 和 Verlinde 不变量，推广 Segre–Verlinde 对应并提出相关对称性猜想。
Virtual K-theoretic invariants of the nested Hilbert scheme on $\mathbb{C}^2$
通过构造非交换Hilbert概形的嵌套版本，将$\mathbb{C}^2$上嵌套Hilbert概形表示为向量丛截面的零轨迹，赋予其完美障碍理论，并利用遗忘映射的推出和局部化，推导出等变虚拟欧拉示性数的多元生成级数闭公式。
We compute the intersection Betti numbers of the GIT model of the moduli space of numerically polarized Enriques surfaces of degree 2. The strategy of the cohomological calculation relies on a general method developed by Kirwan to compute the cohomology of GIT quotients of projective varieties, based on the equivariantly perfect stratification of the unstable points studied by Hesselink and others and a partial resolution of singularities, called Kirwan blow-up.
Virtual invariants of critical loci in GIT quotients of linear spaces
本文利用Jeffrey-Kirwan局部化公式的等变版本，推导了线性空间GIT商中临界轨迹的虚拟不变量公式，并恢复了物理学家预测的箭图表示模空间中的相关结果。
Characterizing Multigraded Regularity and Virtual Resolutions on Products of Projective Spaces
通过截断的极小分次自由分解与Betti数，给出了乘积射影空间上模的多分次Castelnuovo-Mumford正则区域的新刻画与上界，并应用于完全交。
We use virtual neighborhood technique to establish GW-invariants, Quantum cohomology, equivariant GW-invariants, equivariant quantum cohomology and Floer cohomology for general symplectic manifold. We also establish GW-invariants for a family of symplectic manifolds. As a consequence, we prove Arnold conjecture for nondegenerate Hamiltonian symplectomorphisms.
For a proper scheme X with a fixed 1-perfect obstruction theory, we define virtual versions of holomorphic Euler characteristic, chi y-genus, and elliptic genus; they are deformation invariant, and extend the usual definition in the smooth case. We prove virtual versions of the Grothendieck-Riemann-Roch and Hirzebruch-Riemann-Roch theorems. We show that the virtual chi y-genus is a polynomial, and use this to define a virtual topological Euler characteristic. We prove that the virtual elliptic genus satisfies a Jacobi modularity property; we state and prove a localization theorem in the toric …
We continue the approach toward a purely combinatorial "virtual" intersection cohomology for possibly non-rational fans, based on our investigation of equivariant intersection cohomology for toric varieties (see math.AG/9904159). Fundamental objects of study are "minimal extension sheaves" on "fan spaces". These are flabby sheaves of graded modules over a sheaf of polynomial rings, satisfying three relatively simple axioms that characterize the properties of the equivariant intersection cohomology sheaf on a toric variety, endowed with the finite topology given by open invariant subsets. These…
Betti bounds for spaces of curves on varieties and Manin's conjecture for quartic del Pezzo surfaces
该研究证明了射影簇上固定亏格曲线态射空间的紧支撑贝蒂数的一致指数界，并将其应用于建立分裂四次德尔佩佐曲面马宁猜想的高亏格函数域版本。
Virtual Localisation Formula for $ SL_η $-Oriented Theories
本文将 Levine 虚拟局部化公式推广至 motivic 环谱类，获得 Witt 理论中虚拟基本类的局部化公式，并建立等变 Atiyah-Bott 局部化定理及 $N$-等变虚拟局部化公式，同时描述 $\mathrm{KW}(BN)$ 的环结构。
Note on Euler characteristic of a toric vector bundle
本文基于凸链与托尔等变向量束的关系，证明凸链在格点上的求和等于向量束的欧拉特征。
This is the second of series of papers studyig moduli spaces of a certain class of coherent sheaves, which we call stable perverse coherent sheaves, on the blow-up of a projective surface at a point. The followings are main results of this paper: a) We describe the wall-crossing between moduli spaces caused by twisting of the line bundle associated with the exceptional divisor. b) We give the formula for virtual Hodge numbers of moduli spaces of stable perverse coherent sheaves. Moreover we also give proofs of the followings which we observed in a special case in arXiv:0802.3120: c) The moduli…
Using the equivariant virtual cycle of the moduli space of stable maps to [C/Z_r], or equivalently, the vanishing of high-degree Chern classes of a certain vector bundle over the moduli space of stable maps to BZ_r, we derive relations in the Chow ring of Mbar_{g,n}(BZ_r,0). These push forward to yield tautological relations on Mbar_{g,n}.
ADHM invariants are equivariant virtual invariants of moduli spaces of twisted cyclic representations of the ADHM quiver in the abelian category of coherent sheaves of a smooth complex projective curve X. The goal of the present paper is to present a generalization of this construction employing a more general stability condition which depends on a real parameter. This yields a chamber structure in the ADHM theory of curves, residual ADHM invariants being defined by equivariant virtual integration in each chamber. Wallcrossing results and applications to local stable pair invariants will be pr…
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