A characterization of certain irreducible symplectic 4-folds

On the log minimal model program for irreducible symplectic varieties

We show the termination of any log-minimal model program for a pair $(X,Δ)$ of a symplectic manifold $X$ and an effective $\mathbb R$-divisor $Δ$.

Base divisors of big and nef line bundles on irreducible symplectic varieties

Under some conditions on the deformation type, which we expect to be satisfied for arbitrary irreducible symplectic varieties, we describe which big and nef line bundles on irreducible symplectic varieties have base divisors. In particular, we show that such base divisors are always irreducible and reduced. This is applied to understand the behaviour of divisorial base components of big and nef line bundles under deformations and for K3$^{[n]}$-type and Kum$^n$-type.

On monodromies of a degeneration of irreducible symplectic Kähler manifolds

We study the monodromy operators on the betti cohomologies associated to a good degeneration of irreducible symplectic manifold and we show that the unipotency of the monodromy operator on the middle cohomology is at least the half of the dimension.

Small Contractions of Symplectic 4-folds

We classify small contractions of (holomorphically) symplectic 4-folds.

Lattice polarized irreducible holomorphic symplectic manifolds

We generalize Nikulin's and Dolgachev's lattice-theoretical mirror symmetry for K3 surfaces to lattice polarized higher dimensional irreducible holomorphic symplectic manifolds. In the case of fourfolds of $K3^{\left[2\right]}-$type we then describe mirror families of polarized fourfolds and we give an example with mirror non-symplectic involutions.

On the existence of universal families of marked irreducible holomorphic symplectic manifolds

We prove the existence of a universal family over every component of the moduli space of marked irreducible holomorphic symplectic manifolds. The analogous result follows for the Teichmuller spaces.

On the definition of irreducible holomorphic symplectic manifolds and their singular analogs

In the definition of irreducible holomorphic symplectic manifolds the condition of being simply connected can be replaced by vanishing irregularity. We discuss finite quotients X of complex tori where the space of reflexive 2-forms is generated by a holomorphic symplectic form, and their Lagrangian fibrations. Neither X nor the base can be smooth unless X is a 2-torus.

On the birational geometry for irreducible symplectic 4-folds related to the Fano schemes of lines

There is two group actions on the Fano scheme of lines such that the quotient becomes an irreducible symplectic manifold. We showed that both quotients are birational to the generalized Kummer variety or the 2-points Hilbert scheme of a K3 surface.

Birational geometry of irreducible holomorphic symplectic tenfolds of O'Grady type

In this paper we analyse the birational geometry of O'Grady ten dimensional manifolds, giving a characterisation of Kaehler classes and lagrangian fibrations. Moreover, we study symplectic compactifications of intermediate jacobian fibrations of smooth cubic fourfolds.

On isotropic divisors on irreducible symplectic manifolds

Let X be an irreducible symplectic manifold and L a divisor on X. Assume that L is isotropic with respect to the Beauville-Bogomolov quadratic form. We define the rational Lagrangian locus and the movable locus on the universal deformation space of the pair (X, L). We prove that the rational Lagrangian locus is empty or coincide with the movable locus.

Topological invariants of O'Grady's six dimensional irreducible symplectic variety

We study O'Grady examples of irreducible symplectic varieties: we establish that both of them can be deformed into lagrangian fibrations. We analyse in detail the topology of the six dimensional example: in particular we compute its Euler characteristic and determine its Beauville form.

On the monodromy of irreducible symplectic manifolds

Exploiting recent results on the ample cone of irreducible symplectic manifolds, we provide a different point of view for the computation of their monodromy groups. In particular, we give the final step in the computation of the monodromy group for generalised Kummer manifolds and we prove that the monodromy of O'Grady's ten dimensional manifold is smaller than what was expected.

Examples of irreducible symplectic varieties

Irreducible symplectic manifolds are one of the three building blocks of compact Kähler manifolds with numerically trivial canonical bundle by the Beauville-Bogomolov decomposition theorem. There are several singular analogues of irreducible symplectic manifolds, in particular in the context of compact Kähler orbifolds, and in the context of normal projective varieties with canonical singularities. In this paper we will collect their definitions, analyze their mutual relations and provide a list of known examples.

Wall divisors on irreducible symplectic orbifolds of Nikulin-type

We determine the wall divisors on irreducible symplectic orbifolds which are deformation equivalent to a special type of examples, called Nikulin orbifolds. The Nikulin orbifolds are obtained as partial resolutions in codimension 2 of a quotient by a symplectic involution of a Hilbert scheme of 2 points on a K3 surface. This builds on the previous article arXiv:2009.04873 in which the theory of wall divisors was generalized to orbifold singularities.

Moduli spaces of polarized irreducible symplectic manifolds are not necessarily connected

We show that the moduli space of polarized irreducible symplectic manifolds of $K3^{[n]}$-type, of fixed polarization type, is not always connected. This can be derived as a consequence of Eyal Markman's characterization of polarized parallel-transport operators of $K3^{[n]}$-type.

On the Chow ring of birational irreducible symplectic varieties

We show that the graded Chow rings of two birational irreducible symplectic varieties are isomorphic. This lifts a result known for the cohomology algebras to the level of Chow rings, despite the non-injectivity the cycle class map. In the special case of general Mukai flops, we present an alternative approach based on explicit calculations.

通过K3-del Pezzo双重覆盖构造不可约辛簇

Irreducible symplectic varieties via K3-del Pezzo double covers

对于2≤n≤10，构造第二Betti数为16≤b₂≤24的2n维不可约辛簇，作为一般K3-del Pezzo双重覆盖上Beauville-Mukai系统的有限辛商群的非平凡终端化。

Global Torelli theorem for irreducible symplectic orbifolds

We propose a generalization of Verbitsky's global Torelli theorem in the framework of compact Kähler irreducible holomorphically symplectic orbifolds by adapting Huybrechts' proof (arXiv:1106.5573). As intermediate step needed, we also provide a generalization of the twistor space and the projectivity criterion based on works of Campana (arXiv:math/0402243) and Huybrechts (arXiv:math/0106014) respectively.

不可约辛簇的Fujiki关系与纤维化

Fujiki relations and fibrations of irreducible symplectic varieties

本文针对推广不可约辛流形的奇异复射影簇，证明广义Beauville-Bogomolov形式满足Fujiki关系且秩与光滑情形一致，并用新定义验证其纤维化表现与不可约辛流形相似。

通过相对 Prym 簇构造不可约辛簇

Irreducible symplectic varieties via relative Prym varieties

本文利用带反辛对合的 K3 曲面及商曲面线性系构造相对 Prym 簇，给出其为本原或不可约辛簇的判据，并在无穷多维度中产生不可约辛簇序列。

具有较大第二Betti数的不可约辛簇

Irreducible symplectic varieties with a large second Betti number

本文证明了非紧Lagrangian fibration存在不可约辛紧化，并应用于证明包含固定cubic fourfold的cubic fivefolds的relative Jacobian fibration可被第二Betti数至少为24的不可约辛簇紧化，属于新形变类型。

Connected components of moduli spaces of irreducible holomorphic symplectic manifolds of Kummer type

Moduli spaces of polarised (with fixed polarisation type) irreducible holomorphic symplectic manifolds deformation equivalent to Hilbert schemes of points on $K3$ surfaces are not connected in general and A. Apostolov counted the number of their components. We extend Apostolov's results to the case of manifolds deformation equivalent to generalised Kummer varieties.

Irreducible symplectic varieties from moduli spaces of sheaves on K3 and Abelian surfaces

We show that the moduli spaces of sheaves on a projective K3 surface are irreducible symplectic varieties, and that the same holds for the fibers of the Albanese map of moduli spaces of sheaves on an Abelian surface.

关于不可约辛轨形上的Kähler锥

On the Kähler cone of irreducible symplectic orbifolds

本文在不可约辛轨形上推广了全局Torelli定理及Kähler锥相关结果，并新定义了镜像对称作为模空间上的对合。

Base manifolds for fibrations of projective irreducible symplectic manifolds

Given a projective irreducible symplectic manifold $M$ of dimension $2n$, a projective manifold $X$ and a surjective holomorphic map $f:M \to X$ with connected fibers of positive dimension, we prove that $X$ is biholomorphic to the projective space of dimension $n$. The proof is obtained by exploiting two geometric structures at general points of $X$: the affine structure arising from the action variables of the Lagrangian fibration $f$ and the structure defined by the variety of minimal rational tangents on the Fano manifold $X$.

不可约辛簇的扭曲的有限性

On the finiteness of twists of irreducible symplectic varieties

本文利用不可约辛簇的锥猜想，证明了在特征0的固定有限域扩张下不可约辛簇扭曲的有限性，并讨论了非闭域锥猜想、导出等价扭曲有限性及正特征情形。

Deformation theory of singular symplectic n-folds

The notion of a holomorphically symplectic manifold can be generalized to the singular one. This paper studies the birational contraction maps between symplectic varieties, and then describes the deformation of a symplectic variety which has a symplectic resolution. Some examples are also included.

Wall divisors and algebraically coisotropic subvarieties of irreducible holomorphic symplectic manifolds

Rational curves on Hilbert schemes of points on $K3$ surfaces and generalised Kummer manifolds are constructed by using Brill-Noether theory on nodal curves on the underlying surface. It turns out that all wall divisors can be obtained, up to isometry, as dual divisors to such rational curves. The locus covered by the rational curves is then described, thus exhibiting algebraically coisotropic subvarieties that deform to general small deformations of the manifold. This provides strong evidence for a conjecture by Voisin concerning the Chow ring of irreducible holomorphic symplectic manifolds. …

Induced Automorphisms on Irreducible Symplectic Manifolds

We introduce the notion of induced automorphisms in order to state a criterion to determine whether a given automorphism on a manifold of $K3^{[n]}$ type is, in fact, induced by an automorphism of a $K3$ surface and the manifold is a moduli space of stable objects on the $K3$. This criterion is applied to the classification of non-symplectic prime order automorphisms on manifolds of $K3^{[2]}$ type and we prove that almost all cases are covered. Variations of this notion and the above criterion are introduced and discussed for the other known deformation types of irreducible symplectic manifol…

已知类型的不可约辛簇的超奇异Tate猜想：I

Supersingular Tate conjecture for irreducible symplectic varieties of known types: I

本文证明多种已知类型的超奇异不可约辛簇在特定Artin不变量条件下具有Tate或超奇异阿贝尔Chow motive，进而其乘积满足超奇异Tate猜想。

不可约全纯辛流形nef锥的初等描述

An elementary description of nef cone for irreducible holomorphic symplectic manifolds

该研究描述K3型和Kummer型不可约全纯辛流形的MBM类，分析其与相关理论的联系，并将低维数值方法应用于这两类情况，为nef锥研究提供基础结果。

Counter-example to global Torelli problem for irreducible symplectic manifolds

We shall give an example of irreducible symplectic manifolds X and Y which are not bimeromorphic, but have the same periods in the second cohomologies. More explicitly, X and Y are generalized Kummer varieties of dim 4 for a general complex torus T of dim 2 and its dual torus T^*. A key result is an observation of Shioda for the periods of T and T^*. As a concluding remark, we shall discuss the difference between our case and Kummer surfaces.

On subgroups of an automorphism group of an irreducible symplectic manifold

Let X be an irreducible symplectic manifold and L a nef line bundle on X which is isotropic with respect to the Beauville-Bogomolov quadratic form. It is known that a subgroup Aut(X,L) of an automorphism group of X which fix L is almost abelian. We give a formula of the rank of Aut(X,L) in terms of MBM divisors. We also prove that the nef cone of X cut out MBM classes, which is a generalization of Kovac's structure theorem of nef cones of K3 surfaces

Smith theory and irreducible holomorphic symplectic manifolds

We study the cohomological properties of the fixed locus $X^G$ of an automorphism group $G$ of prime order $p$ acting on a variety $X$ whose integral cohomology is torsion-free. We obtain an precise relation between the mod $p$ cohomology of $X^G$ and natural invariants for the action of $G$ on the integral cohomology of $X$. We apply these results to irreducible holomorphic symplectic manifolds of deformation type of the Hilbert scheme of two points on a K3 surface: the main result of this paper is a formula relating the dimension of the mod $p$ cohomology of $X^G$ with the rank and the discr…

Brane involutions on irreducible holomorphic symplectic manifolds

In the context of irreducible holomorphic symplectic manifolds, we say that (anti)holomorphic (anti)symplectic involutions are brane involutions since their fixed point locus is a brane in the physicists' language, i.e. a submanifold which is either complex or lagrangian submanifold with respect to each of the three Kähler structures of the associated hyperkähler structure. Starting from a brane involution on a K3 or abelian surface, one can construct a natural brane involution on its moduli space of sheaves. We study these natural involutions and their relation with the Fourier--Mukai transfo…

On Irreducible Symplectic Varieties of $\mathrm{K3}^{[n]}$-type in Positive Characteristic

We show that there is a good notion of irreducible sympelectic varieties of $\mathrm{K3}^{[n]}$-type over an arbitrary field of characteristic zero or $p > n + 1$. Then we construct mixed characteristic moduli spaces for these varieties. Our main result is a generalization of Ogus' crystalline Torelli theorem for supersingular K3 surfaces. For applications, we answer a slight variant of a question asked by F. Charles on moduli spaces of sheaves on K3 surfaces and give a crystalline Torelli theorem for supersingular cubic fourfolds.

关于不可约辛流形的Boucksom-Zariski分解与负数有界性

On the Boucksom-Zariski decomposition for irreducible symplectic varieties and bounded negativity

本文研究了不可约辛流形上Boucksom-Zariski分解的性质，并证明了在光滑情况下负数有界猜想的有效版本，从而得出关于辛流形上大线包的有效应性结论。

带有 $\mathbb Z^4_3 : \mathcal A_6$ 作用的不可约全纯辛流形

Irreducible Holomorphic Symplectic manifolds with an action of $\mathbb Z^4_3 : \mathcal A_6$

本文刻画了同时具有最大阶辛群 $\mathbb Z_3^4 : \mathcal A_6$ 作用和非辛自同构的 K3$^{[2]}$ 型不可约全纯辛流形，并证明 Fermat 三次四维流形的直线 Fano 簇达到自同构群最大阶 174960。

On fibre space structures of a projective irreducible symplectic manifold

In this note, we investigate fibre space structures of a projective irreducible symplectic manifold. We prove that an 2n-dimensional projective irreducible symplectic manifold admits only an n-dimensional fibration over a Fano variety which has only Q-factorial log-terminal singularities and whose Picard number is one. Moreover we prove that a general fibre is an abelian variety up to finite unramified cover, especially, a general fibre is an abelian surface for 4-fold.

Analytic torsion for irreducible holomorphic symplectic fourfolds with involution, II: the singularity of the invariant (with an Appendix by Ken-Ichi Yoshikawa)

We study the boundary behavior of the invariant of $K3^{[2]}$-type manifolds with antisymplectic involution, which we obtained using equivariant analytic torsion. We show the algebraicity of the singularity of the invariant by using the asymptotic of equivariant Quillen metrics and equivariant $L^2$-metrics. We prove that, in some cases, the invariant coincides with Yoshikawa's invariant for 2-elementary K3 surfaces. Hence, in these cases, our invariant is expressed as the Petersson norm of a Borcherds product and a Siegel modular form.

一个辛四维流形

A symplectic fourfold

本文通过研究超Kähler流形非自然群作用的终端化，构造了具有b2=4和非商奇点的四维不可约辛流形，为已知全球Torelli定理不成立的ISV提供了具体例子。

A new six dimensional irreducible symplectic variety

By results of the author there exists a projective (holomorphic) symplectic desingularization of the moduli space of rank-two torsion-free sheaves on a genus-two Jacobian with $c_1=0$ and $c_2=2$. This desingularization has a natural map to the self-product of the Jacobian. We show that the fiber over $(0,0)$ is a 6-dimensional projective irreducible symplectic variety (and hence a 12-dimensional compact Hyperkahler manifold) with second Betti number equal to 8. Thus it is not deformation equivalent to any of the (few) known examples of irreducible symplectic varieties, even up to birational e…

Density of Noether-Lefschetz loci of polarized irreducible holomorphic symplectic varieties and applications

In this note we derive from deep results due to Clozel-Ullmo the density of Noether-Lefschetz loci inside the moduli space of marked (polarized) irreducible holomorphic symplectic (IHS) varieties. In particular we obtain the density of Hilbert schemes of points on projective $K3$ surfaces and of projective generalized Kummer varieties in their moduli spaces. We present applications to the existence of rational curves on projective deformations of such varieties, to the study of relevant cones of divisors, and a refinement of Hassett's result on cubic fourfolds whose Fano variety of lines is is…

Symplectic geometry and rationally connected 4-folds

We study some symplectic geometric aspects of rationally connected 4-folds. As a corollary, we prove that any smooth projective 4-fold symplectic deformation equivalent to a Fano 4-fold of pseudo-index at least 2 or a rationally connected 4-fold whose second Betti number is 2 is rationally connected.

具有对合的不可约全纯辛四重形的解析挠率，I：一个不变量的构造

Analytic torsion for irreducible holomorphic symplectic fourfolds with involution, I: Construction of an invariant

本文利用等变解析挠率，为具有反辛对合的 $K3^{[2]}$ 型不可约全纯辛流形构造了一个新不变量，并给出了该不变量对数的复 Hessian 公式。

Moduli spaces of irreducible symplectic manifolds

We study the moduli spaces of polarised irreducible symplectic manifolds. By a comparison with locally symmetric varieties of orthogonal type of dimension 20, we show that the moduli space of 2d polarised (split type) symplectic manifolds which are deformation equivalent to degree 2 Hilbert schemes of a K3 surface is of general type if d is at least 12.

Boucksom-Zariski 与 Weyl 腔室在不可约全纯辛流形上的研究

Boucksom-Zariski and Weyl chambers on irreducible holomorphic symplectic manifolds

受 Bauer 等人启发，本文为射影不可约全纯辛流形的大锥建立腔室分解，并研究其与 Weyl 腔室及体积函数的关系，最终确定伪有效锥结构。

On the primitivity of birational transformations of irreducible symplectic manifolds

Let $f\colon X\dashrightarrow X$ be a bimeromorphic transformation of a complex irreducible symplectic manifold $X$. Some important dynamical properties of $f$ are encoded by the induced linear automorphism $f^*$ of $H^2(X,\mathbb Z)$. Our main result is that a bimeromorphic transformation such that $f^*$ has at least one eigenvalue with modulus $>1$ doesn't admit any invariant fibration (in particular its generic orbit is Zariski-dense).

On intersection cohomology and Lagrangian fibrations of irreducible symplectic varieties

We prove several results concerning the intersection cohomology and the perverse filtration associated with a Lagrangian fibration of an irreducible symplectic variety. We first show that the perverse numbers only depend on the deformation equivalence class of the ambient variety. Then we compute the border of the perverse diamond, which further yields a complete description of the intersection cohomology of the Lagrangian base and the invariant cohomology classes of the fibers. Lastly, we identify the perverse and Hodge numbers of intersection cohomology when the irreducible symplectic variet…

Supersingular irreducible symplectic varieties

We study symplectic varieties defined over fields of positive characteristics, especially the supersingular ones, generalizing the theory of supersingular K3 surfaces. In this work, we are mainly interested in the following two types of symplectic varieties over an algebraically closed field of positive characteristic, under natural numerical conditions: (1) smooth moduli spaces of sheaves on K3 surfaces and (2) smooth Albanese fibers of moduli spaces of sheaves on abelian surfaces. Several natural definitions of the supersingularity for symplectic varieties are discussed, which are proved to …

Riemann-Roch多项式、MBM类与贫乏IHS流形

Riemann-Roch Polynomials, MBM Classes and Poor IHS Manifolds

本文研究贫乏不可约全纯辛流形，证明其自然锥重合且无全纯叶理，并利用Riemann-Roch多项式根给出检测MBM类及有理曲线存在的统一准则，同时刻画了此类流形的形变空间。

Kuznetsov分支上的模空间是不可约辛簇

Moduli spaces on Kuznetsov components are Irreducible Symplectic Varieties

本文研究立方四胞体Kuznetsov分支中具非本原非OG10型Mukai向量的Bridgeland半稳定对象模空间，证明其为具因子终端奇点的射影不可约辛簇，形变类由m和v₀²唯一确定，推广并拓展了此前相关模空间的研究结果。

On irreducible symplectic 4-folds numerically equivalent to $(K3)^{[2]}$

We address the problem of classification of hyper-Kähler fourfolds with $b_2=23$. In particular we prove some special cases of the Conjecture of O'Grady about hyper-Kähler $4$-folds numerically equivalent to the Hilbert scheme of two points on a $K3$ surface.

Lagrangian fibrations on symplectic fourfolds

We prove that there are at most two possibilities for the base of a Lagrangian fibration from a complex projective irreducible symplectic fourfold.