Anticanonical Divisor Research Articles

Examples of Fano manifolds with non‐pseudoeffective tangent bundle

Let X $X$ be a Fano manifold. While the properties of the anticanonical divisor − K X $-K_X$ and its multiples have been studied by many authors, the positivity of the tangent bundle T X $T_X$ is much more elusive. We give a complete characterisation of the pseudoeffectivity of T X $T_X$ for del Pezzo surfaces, hypersurfaces in the projective space and del Pezzo threefolds.

Pathologies and liftability of Du Val del Pezzo surfaces in positive characteristic

In this paper, we study pathologies of Du Val del Pezzo surfaces defined over an algebraically closed field of positive characteristic by relating them to their non-liftability to the ring of Witt vectors. More precisely, we investigate the condition (NB): all the anti-canonical divisors are singular, (ND): there are no Du Val del Pezzo surfaces over the field of complex numbers with the same Dynkin type, Picard rank, and anti-canonical degree, (NK): there exists an ample \(\mathbb {Z}\)-divisor which violates the Kodaira vanishing theorem for \(\mathbb {Z}\)-divisors, and (NL): the pair (Y, E) does not lift to the ring of Witt vectors, where Y is the minimal resolution and E is its reduced exceptional divisor. As a result, for each of these conditions, we determine all the Du Val del Pezzo surfaces which satisfy the given one.

Projective flatness over klt spaces and uniformisation of varieties with nef anti-canonical divisor

We give a criterion for the projectivisation of a reflexive sheaf on a klt space to be induced by a projective representation of the fundamental group of the smooth locus. This criterion is then applied to give a characterisation of finite quotients of projective spaces and Abelian varieties byQ\mathbb {Q}-Chern class (in)equalities and a suitable stability condition. This stability condition is formulated in terms of a naturally defined extension of the tangent sheaf by the structure sheaf. We further examine cases in which this stability condition is satisfied, comparing it to K-semistability and related notions.

Positivity of anticanonical divisors in algebraic fibre spaces

Let $$f:X\rightarrow Y$$ be an algebraic fibre space between normal projective varieties and F be a general fibre of f. We prove an Iitaka-type inequality $$\kappa (X,-K_X)\le \kappa (F,-K_F)+\kappa (Y,-K_Y)$$ under some mild conditions. We also obtain results relating the positivity of $$-K_X$$ and $$-K_Y$$ .

Kawaguchi–Silverman conjecture for certain surjective endomorphisms

We prove the Kawaguchi–Silverman conjecture (KSC), about the equality of arithmetic degree and dynamical degree, for every surjective endomorphism of any (possibly singular) projective surface. In high dimensions, we show that KSC holds for every surjective endomorphism of any \mathbb{Q} -factorial Kawamata log terminal projective variety admitting one int-amplified endomorphism, provided that KSC holds for any surjective endomorphism with the ramification divisor being totally invariant and irreducible. In particular, we show that KSC holds for every surjective endomorphism of any rationally connected smooth projective threefold admitting one int-amplified endomorphism. The main ingredients are the equivariant minimal model program, the effectiveness of the anti-canonical divisor and a characterization of toric pairs.

Structure of projective varieties with nef anticanonical divisor: the case of log terminal singularities

In this article we study the structure of klt projective varieties with nef anticanonical divisor (and more generally, varieties of semi-Fano type), especially the canonical fibrations associated to them. We show that The Albanese map for such variety is a locally constant fibration (that is, a locally trivial fibration such that X is isomorphic to the product of the universal cover of the Albanese torus by the fibre of the Albanese map quotient by a diagonal action of the fundamenatl group of the Albanese torus). If the smooth locus is simply connected, the MRC fibration of such variety is an everywhere defined morphism and induces a decomposition of this variety into a product of a rationally connected variety by a projective variety with trivial canonical divisor. These generalize the corresponding results for smooth projective varieties with nef anticanonical bundle in Cao (2019) and Cao-Höring (2019) to the klt case, and partially reduce the structure problem of these varieties to the singular Beauville-Bogomolov decomposition theorem proved by successive works of Greb-Kebekus-Peternell (2016), Druel (2018), Guenencia-Greb-Kebekus (2019) and Höring-Peternell (2019).

Stable maps to Looijenga pairs: orbifold examples

In arXiv:2011.08830 we established a series of correspondences relating five enumerative theories of log Calabi-Yau surfaces, i.e. pairs $(Y,D)$ with $Y$ a smooth projective complex surface and $D=D_1+\dots +D_l$ an anticanonical divisor on $Y$ with each $D_i$ smooth and nef. In this paper we explore the generalisation to $Y$ being a smooth Deligne-Mumford stack with projective coarse moduli space of dimension 2, and $D_i$ nef $\mathbb{Q}$-Cartier divisors. We consider in particular three infinite families of orbifold log Calabi-Yau surfaces, and for each of them we provide closed form solutions of the maximal contact log Gromov-Witten theory of the pair $(Y,D)$, the local Gromov-Witten theory of the total space of $\bigoplus_i \mathcal{O}_Y(-D_i)$, and the open Gromov-Witten theory of toric orbi-branes in a Calabi-Yau 3-orbifold associated to $(Y,D)$. We also consider new examples of BPS integral structures underlying these invariants, and relate them to the Donaldson-Thomas theory of a symmetric quiver specified by $(Y,D)$, and to a class of open/closed BPS invariants.

Projective klt pairs with nef anti-canonical divisor

In this paper, we study a projective klt pair $(X, \Delta)$ with the nef anti-log canonical divisor $-(K_X+\Delta)$ and its maximally rationally connected fibration $\psi: X \dashrightarrow Y$. We prove that the numerical dimension of the anti-log canonical divisor $-(K_X+\Delta)$ on $X$ coincides with that of the anti-log canonical divisor $-(K_{X_y}+\Delta_{X_y})$ on a general fiber $X_y$ of $\psi: X \dashrightarrow Y$, which is an analogue of Ejiri-Gongyo's result formulated for the Kodaira dimension. As a corollary, we reveal a relation between positivity of the anti-canonical divisor and the rational connectedness, which gives a sharper estimate than the question posed by Hacon-$\mathrm{M^{c}}$Kernan. Moreover, in the case of $X$ being smooth, we show that a maximally rationally connected fibration $\psi: X \to Y$ can be chosen to be a morphism to a smooth projective variety $Y$ with numerically trivial canonical divisor, and further that it is locally trivial with respect to the pair $(X, \Delta)$, which can be seen as a generalization of Cao-Horing's structure theorem to klt pair cases. Finally, we study the structure of the slope rationally connected quotient for a pair $(X, \Delta)$ with $-(K_X +\Delta)$ nef, and obtain a structure theorem for projective orbifold surfaces.

Smoothings and rational double point adjacencies for cusp singularities

A cusp singularity is a surface singularity whose minimal resolution is a cycle of smooth rational curves meeting transversely. Cusp singularities come in naturally dual pairs. Looijenga proved in 1981 that if a cusp singularity is smoothable, the minimal resolution of the dual cusp is the anticanonical divisor of some smooth rational surface. In 1983, the second author and Miranda gave a criterion for smoothability of a cusp singularity, in terms of the existence of a K-trivial semistable model for the central fiber of such a smoothing. We study these "Type III degenerations" of rational surfaces with an anticanonical divisor--their deformations, birational geometry, and monodromy. Looijenga's original paper also gave a description of the rational double point configurations to which a cusp singularity deforms, but only in the case where the resolution of the dual cusp has cycle length 5 or less. We generalize this classification to an arbitrary cusp singularity, giving an explicit construction of a semistable simultaneous resolution of such an adjacency. The main tools of the proof are (1) formulas for the monodromy of a Type III degeneration, (2) a construction via surgeries on integral-affine surfaces of a degeneration with prescribed monodromy, (3) surjectivity of the period map for Type III central fibers, and (4) a theorem of Shepherd-Barron producing the simultaneous contraction to the adjacency of the cusp singularity.

On the anti-canonical geometry of weak ℚ-Fano threefolds II

By a canonical (resp. terminal) weak $\mathbb{Q}$-Fano $3$-fold we mean a normal projective one with at worst canonical (resp. terminal) singularities on which the anti-canonical divisor is $\mathbb{Q}$-Cartier, nef and big. For a canonical weak $\mathbb{Q}$-Fano $3$-fold $V$, we show that there exists a terminal weak $\mathbb{Q}$-Fano $3$-fold $X$, being birational to $V$, such that the $m$-th anti-canonical map defined by $|-mK_{X}|$ is birational for all $m\geq 52$. As an intermediate result, we show that for any $K$-Mori fiber space $Y$ of a canonical weak $\mathbb{Q}$-Fano $3$-fold, the $m$-th anti-canonical map defined by $|-mK_{Y}|$ is birational for all $m\geq 52$.

Tropical Lagrangians in toric del-Pezzo surfaces

We look at how one can construct from the data of a dimer model a Lagrangian submanifold in (mathbb {C}^*)^n whose valuation projection approximates a tropical hypersurface. Each face of the dimer corresponds to a Lagrangian disk with boundary on our tropical Lagrangian submanifold, forming a Lagrangian mutation seed. Using this we find tropical Lagrangian tori L_{T^2} in the complement of a smooth anticanonical divisor of a toric del-Pezzo whose wall-crossing transformations match those of monotone SYZ fibers. An example is worked out for the mirror pair (mathbb {CP}^2{setminus } E, W), {check{X}}_{9111}. We find a symplectomorphism of mathbb {CP}^2{setminus } E interchanging L_{T^2} and a SYZ fiber. Evidence is provided that this symplectomorphism is mirror to fiberwise Fourier–Mukai transform on {check{X}}_{9111}.

Moduli of double covers and degree one del Pezzo surfaces

Given a degree one del Pezzo surface with canonical singularities, the linear series generated by twice the anti-canonical divisor exhibits the surface as the double cover of the quadric cone branched along a sextic curve. It is natural to ask if this description extends to the boundary of a compactification of the moduli space of degree one del Pezzo surfaces. The goal of this paper is to show that this is indeed the case. In particular, we give an explicit classification of the boundary of the moduli space of anti-canonically polarized broken del Pezzo surfaces of degree one as double covers of degenerations of the quadric cone.

On ruled surfaces with big anti-canonical divisor and numerically trivial divisors on weak log Fano surfaces

We investigate the structure of geometrically ruled surfaces whose anti-canonical class is big. As an application we show that the Picard group of a normal projective surface whose anti-canonical class is nef and big is a free abelian group of finite rank.

Platonic Harbourne-Hirschowitz Rational Surfaces

The aim of this work was to study the finite generation of the effective monoid and Cox ring of a Platonic Harbourne-Hirschowitz rational surface with an anticanonical divisor not reduced which contains some exceptional curves as irreducible components. Such surfaces are obtained as the blow up of the n-Hirzebruch surface at any number of points lying in the union of the negative section and $$n+2$$ different fibers. Moreover, the procedure that ensures the finite generation of the effective monoid provides a technique for explicit computation of the minimal generating set for such monoid in concrete cases. As an application, we present explicitly the minimal generating set for the effective monoid of some surfaces which are obtained by considering a degenerate cubic consisting in three lines intersecting at one point in the projective plane and blowing-up the singular point and some ordinary and infinitely near points. The base field of our surfaces is assumed to be algebraically closed of arbitrary characteristic.

Seshadri constants of the anticanonical divisors of Fano manifolds with large index

Seshadri constants, introduced by Demailly, measure the local positivity of a nef divisor at a point. In this paper, we compute the Seshadri constants of the anticanonical divisors of Fano manifolds with coindex at most 3 at a very general point. As a consequence, if X is a nonsingular Fano threefold which is very general in its deformation family, then ε(X,−KX;x)≤1 for all points x∈X if and only if |−KX| is not base point free.

The B-model connection and mirror symmetry for Grassmannians

We consider the Grassmannian X=Grn−k(Cn) and describe a ‘mirror dual’ Landau-Ginzburg model (Xˇ∘,Wq:Xˇ∘→C), where Xˇ∘ is the complement of a particular anti-canonical divisor in a Langlands dual Grassmannian Xˇ, and we express W succinctly in terms of Plücker coordinates. First of all, we show this Landau-Ginzburg model to be isomorphic to one proposed for homogeneous spaces in a previous work by the second author. Secondly we show it to be a partial compactification of the Landau-Ginzburg model defined in the 1990's by Eguchi, Hori, and Xiong. Finally we construct inside the Gauss-Manin system associated to Wq a free submodule which recovers the trivial vector bundle with small Dubrovin connection defined out of Gromov-Witten invariants of X. We also prove a T-equivariant version of this isomorphism of connections. Our results imply in the case of Grassmannians an integral formula for a solution to the quantum cohomology D-module of a homogeneous space, which was conjectured by the second author. They also imply a series expansion of the top term in Givental's J-function, which was conjectured in a 1998 paper by Batyrev, Ciocan-Fontanine, Kim and van Straten.

Open Gromov–Witten invariants and mirror maps for semi-Fano toric manifolds

We prove that for a compact toric manifold whose anti-canonical divisor is numerically effective, the Lagrangian Floer superpotential defined by Fukaya-Oh-Ohto-Ono is equal to the superpotential written down by using the toric mirror map under a convergence assumption. This gives a method to compute open Gromov-Witten invariants using mirror symmetry.

Positivity of anticanonical divisors and F-purity of fibers

In this paper, we prove that given a flat generically smooth morphism between smooth projective varieties with $F$-pure closed fibers, if the source space is Fano, weak Fano or a variety with the nef anti-canonical divisor, then so is the target space. We also show that relative anti-canonical divisors of generically smooth surjective nonconstant morphisms are not nef and big in arbitrary characteristic.

On the symplectic cohomology of log Calabi–Yau surfaces

This article studies the symplectic cohomology of affine algebraic surfaces that admit a compactification by a normal crossings anticanonical divisor. Using a toroidal structure near the compactification divisor, we describe the complex computing symplectic cohomology, and compute enough differentials to identify a basis for the degree-zero part of the symplectic cohomology. This basis is indexed by integral points in a certain integral affine manifold, providing a relationship to the theta functions of Gross--Hacking--Keel. Included is a discussion of wrapped Floer cohomology of Lagrangian submanifolds and a description of the product structure in a special case. We also show that, after enhancing the coefficient ring, the degree--zero symplectic cohomology defines a family degenerating to a singular surface obtained by gluing together several affine planes.

Anticanonical system of Fano fivefolds

AbstractWe show that any Fano fivefold with canonical Gorenstein singularities has an effective anticanonical divisor. Moreover, if a general element of the anticanonical system is reduced, then it has canonical singularities.