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Ekaterina Amerik

Publications and source records attributed to Ekaterina Amerik.

At least 19 recordsLinked to original sources
arXiv2026

Reflective lattices and hyperkahler manifolds

Using the results of Nikulin and Vinberg on the groups of isometries generated by reflections, we construct a subvariety called the Nikulin-Vinberg locus in the moduli space of polarized hyperkahler manifolds. It is obtained as a finite union of components of higher Noether-Lefschetz loci which parameterize manifolds with certain special Neron-Severi lattices. The Nikulin-Vinberg locus is the closure of the set of hyperkahler manifolds with Picard number $\geq 3$ which have finite groups of birational automorphisms. Using this construction and a refinement of an argument by Oguiso, we show that any non-trivial family of projective deformations of a hyperkahler manifold with $b_2(M)\geq 6$ has a dense set of fibers which have an infinite group of birational automorphisms.

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arXiv2025

Birational geometry of hyperkahler manifolds and the Hu-Yau conjecture

Wierzba and Wisniewski proved that in dimension 4, every bimeromorphic map of hyperkahler manifolds is represented as a composition of Mukai flops. Hu and Yau conjectured that this result can be generalized to arbitrary dimension. They defined ``Mukai's elementary transformation'' as the blow-up of a subvariety ruled by complex projective spaces, composed with the contraction of the ruling. Hu and Yau conjectured that any bimeromorphic map of hyperkahler manifolds can be decomposed into a sequence of Mukai's elementary transformations, after possibly removing subvarieties of codimension greater than $2$. We prove this conjecture for compact hyperkahler manifolds of maximal holonomy by decomposing any bimeromorphic map into a composition of wall-crossing flops associated with MBM contractions.

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arXiv2025

Finite translation orbits on double families of abelian varieties (with an appendix by E. Amerik)

We study two families of $g$-dimensional abelian varieties, induced by distinct rational maps defined on a common variety $\overline{\mathcal A}$ and mapping to two bases $\overline{S}_1$ and $\overline{S}_2$. Two non-torsion sections induce birational fiberwise translations on $\overline{\mathcal A}$. We consider the action of a specific subset of the group generated by these translations. Under the assumption that $\operatorname{dim} \overline{S}_1 (= \operatorname{dim} \overline{S}_2) \leq g$, we prove that the points with finite orbit are contained in a proper Zariski closed subset. This subset is explicitly described to a certain extent. Our results generalize a theorem of Corvaja, Tsimermann, and Zannier to higher dimensions.

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arXiv2025

Parabolic automorphisms of hyperk{ä}hler manifolds: Orbits and Betti maps

We study parabolic automorphisms of irreducible holomorphically symplectic manifolds with a lagrangian fibration. Such automorphisms are (possibly up to taking a power) fiberwise translations on smooth fibers, and their orbits in a general fiber are dense ([1]). We provide a simple proof that the associated Betti map is of maximal rank, in particular, the set of fibers where the induced translation is of finite order is dense as well. R{É}SUM{É}. Nous {é}tudions les automorphismes paraboliques des vari{é}t{é}s symplectiques holomorphes qui sont irr{é}ductibles et projectives.

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arXiv2024

Apollonian carpets and the boundary of the Kahler cone of a hyperkahler manifold

The ample cone of a compact Kahler $n$-manifold $M$ is the intersection of its Kahler cone and the real subspace generated by integer (1,1)-classes. Its isotropic boundary is the set of all points $η$ on its boundary such that $\int_M η^n=0$. We are interested in the relation between the shape of the isotropic boundary of the ample cone of a hyperkahler manifold and the dynamics of its holomorphic automorphism group $G$. In this case, the projectivization of the ample cone is realized as an open, locally polyhedral subset in a hyperbolic space $H$. The isotropic boundary $S$ is realized as a subset of the hyperbolic boundary (the absolute) $A$ of $H$, which is naturally identified with a Euclidean sphere. It is clear that the isotropic boundary $S$ contains the limit set of $G$ acting on its ample cone. We prove that, conversely, all irrational points on $S$ belong to the limit set. Using a result of N. Shah about limiting distributions of curves under geodesic flow on hyperbolic manifolds, we prove that every real analytic curve in $S$ is contained in a geodesic sphere in $S$,and in presence of such curves the limit set is the closure of the union of these geodesic spheres. We study the geometry of such fractal sets, called Apollonian carpets, and establish the link between the Apollonian carpet and the structure of the automorphism group.

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arXiv2023

On algebraically coisotropic submanifolds of holomorphic symplectic manifolds

We investigate algebraically coisotropic submanifolds $X$ in a holomorphic symplectic projective manifold $M$. Motivated by our results in the hypersurface case, we raise the following question: when $X$ is not uniruled, is it true that up to a finite étale cover, the pair $(X,M)$ is a product $(Z\times Y, N\times Y)$ where $N, Y$ are holomorphic symplectic and $Z\subset N$ is Lagrangian? We prove that this is indeed the case when $M$ is an abelian variety, and give some partial answer when the canonical bundle $K_X$ is semi-ample. In particular, when $K_X$ is nef and big, $X$ is Lagrangian in $M$ (in fact this also holds without nefness assumption). We also remark that Lagrangian submanifolds do not exist on a sufficiently general Abelian variety, in contrast to the case when $M$ is irreducible hyperkähler.

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arXiv2023

Parabolic automorphisms of hyperkahler manifolds

A parabolic automorphism of a hyperkahler manifold is a holomorphic automorphism acting on $H^2(M)$ by a non-semisimple quasi-unipotent linear map. We prove that a parabolic automorphism which preserves a Lagrangian fibration acts on its fibers ergodically. The invariance of a Lagrangian fibration is automatic for manifolds satisfying the hyperkahler SYZ conjecture; this includes all known examples of hyperkahler manifolds. When there are two parabolic automorphisms preserving two distinct Lagrangian fibration, it follows that the group they generate acts on $M$ ergodically. Our results generalize those obtained by S. Cantat for K3 surfaces.

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arXiv2022

Non-linear Hopf manifolds are locally conformally Kahler

A Hopf manifold is a quotient of $C^n\backslash 0$ by the cyclic group generated by a holomorphic contraction. Hopf manifolds are diffeomorphic to $S^1\times S^{2n-1}$ and hence do not admit Kahler metrics. It is known that Hopf manifolds defined by linear contractions (called linear Hopf manifolds) have locally conformally Kahler (LCK) metrics. In this paper we prove that the Hopf manifolds defined by non-linear holomorphic contractions admit holomorphic embeddings into linear Hopf manifolds, and, moreover they admit LCK metrics.

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arXiv2021

Contraction centers in families of hyperkahler manifolds

We study the exceptional loci of birational (bimeromorphic) contractions of a hyperkähler manifold $M$. Such a contraction locus is the union of all minimal rational curves in a collection of cohomology classes which are orthogonal to a wall of the Kähler cone. Homology classes which can possibly be orthogonal to a wall of the Kähler cone of some deformation of $M$ are called MBM classes. We prove that all MBM classes of type (1,1) can be represented by rational curves, called MBM curves. All MBM curves can be contracted on an appropriate birational model of $M$, unless $b_2(M) \leq 5$. When $b_2(M)>5$, this property can be used as an alternative definition of an MBM class and an MBM curve. Using the results of Bakker and Lehn, we prove that the diffeomorphism type of a contraction locus remains stable under all deformations for which these classes remains of type (1,1), unless the contracted variety has $b_2\leq 4$. Moreover, these diffeomorphisms preserve the MBM curves, and induce biholomorphic maps on the contraction fibers, if they are normal.

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arXiv2020

Rational curves and MBM classes on hyperkahler manifolds: a survey

This paper deals with rational curves and birational contractions on irreducible holomorphically symplectic manifold. We survey some recent results about minimal rational curves, their deformations, extremal rays associated with these curves, and the geometry of the Kähler cone.

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arXiv2019

MBM classes and contraction loci on low-dimensional hyperkahler manifolds of K3 type

An MBM locus on a hyperkahler manifold is the union of all deformations of a minimal rational curve with negative self-intersection. MBM loci can be equivalently defined as centers of bimeromorphic contractions. It was shown that the MBM loci on deformation equivalent hyperkahler manifolds are diffeomorphic. We determine the MBM loci on a hyperkahler manifold of K3-type of low dimension using a deformation to a Hilbert scheme of a non-algebraic K3 surface.

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arXiv2019

MBM loci in families of hyperkahler manifolds and centers of birational contractions

An MBM class on a hyperkahler manifold M is a second cohomology class such that its orthogonal complement in H^2(M) contains a maximal dimensional face of the boundary of the Kahler cone for some hyperkahler deformation of M. An MBM curve is a rational curve in an MBM class and such that its local deformation space has minimal possible dimension 2n-2, where 2n is the complex dimension of M. We study the MBM loci, defined as the subvarieties covered by deformations of an MBM curve within M. When M is projective, MBM loci are centers of birational contractions. For each MBM class z, we consider the Teichmuller space $Teich^{min}_z$ of all deformations of M such that $z^{\bot}$ contains a face of the Kahler cone. We prove that for all $I,J\in Teich^{min}_z$, the MBM loci of (M, I) and (M,J) are homeomorphic under a homeomorphism preserving the MBM curves, unless possibly the Picard number of I or J is maximal.

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arXiv2017

Specialness and Isotriviality for Regular Algebraic Foliations

We show that an everywhere regular foliation $\mathcal F$ with compact canonically polarized leaves on a quasi-projective manifold $X$ has isotrivial family of leaves when the orbifold base of this family is special. By a recent work of Berndtsson, Paun and Wang, the same proof works in the case where the leaves have trivial canonical bundle. The specialness condition means that the $p$-th exterior power of the logarithmic extension of its conormal bundle does not contain any rank-one subsheaf of maximal Kodaira dimension $p$, for any $p>0$. This condition is satisfied, for example, in the very particular case when the Kodaira dimension of the determinant of the Logarithmic extension of the conormal bundle vanishes. Motivating examples are given by the `algebraically coisotropic' submanifolds of irreducible hyperkähler projective manifolds.

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arXiv2016

On the characteristic foliation on a smooth hypersurface in a holomorphic symplectic fourfold

Let $X$ be an irreducible holomorphic symplectic fourfold and $D$ a smooth hypersurface in $X$. It follows from a result by Amerik and Campana that the characteristic foliation (that is the foliation given by the kernel of the restriction of the symplectic form to $D$) is not algebraic unless $D$ is uniruled. Suppose now that the Zariski closure of its general leaf is a surface. We prove that $X$ has a lagrangian fibration and $D$ is the inverse image of a curve on its base.

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arXiv2016

Endomorphisms of projective bundles over a certain class of varieties

Let $B$ be a simply-connected projective variety such that the first cohomology groups of all line bundles on $B$ are zero. Let $E$ be a vector bundle over $B$ and $X={\mathbb P} (E)$. It is easily seen that a power of any endomorphism of $X$ takes fibers to fibers. We prove that if $X$ admits an endomorphism which is of degree greater than one on the fibers then $E$ splits into a direct sum of line bundles.

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arXiv2016

Characteristic foliation on non-uniruled smooth divisors on hyperkaehler manifolds

We prove that the characteristic foliation $F$ on a non-singular divisor $D$ in an irreducible projective hyperkaehler manifold $X$ cannot be algebraic, unless the leaves of $F$ are rational curves or $X$ is a surface. More generally, we show that if $X$ is an arbitrary projective manifold carrying a holomorphic symplectic $2$-form, and $D$ and $F$ are as above, then $F$ can be algebraic with non-rational leaves only when, up to a finite étale cover, $X$ is the product of a symplectic projective manifold $Y$ with a symplectic surface and $D$ is the pull-back of a curve on this surface. When $D$ is of general type, the fact that $F$ cannot be algebraic unless $X$ is a surface was proved by Hwang and Viehweg. The main new ingredient for our results is the observation that the canonical bundle of the (apriori, orbifold; but the orbifold structure is actually trivial) base of the family of leaves must be torsion. This implies, in particular, the isotriviality of the family of leaves of $F$. We also make some remarks in the Kähler case and apply this to the Lagrangian conjecture in the last section.

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arXiv2016

Collections of parabolic orbits in homogeneous spaces, homogeneous dynamics and hyperkahler geometry

Let $M$ be a hyperkähler manifold with $b_2(M)\geq 5$. We improve our earlier results on the Morrison-Kawamata cone conjecture by showing that the Beauville-Bogomolov square of the primitive MBM classes (i.e. the classes whose orthogonal hyperplanes bound the Kähler cone in the positive cone, or, in other words, the classes of negative extremal rational curves on deformations of $M$) is bounded in absolute value by a number depending only on the deformation class of $M$. The proof uses ergodic theory on homogeneous spaces.

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