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arXiv2025

Galois descent for motivic theories

We give necessary conditions for a category fibred in pseudo-abelian additive categories over the classifying topos of a profinite group to be a stack; these conditions are sufficient when the coefficients are $\mathbf{Q}$-linear. This applies to pure motives over a field in the sense of Grothendieck, Deligne-Milne and André, to mixed motives in the sense of Nori and to several motivic categories considered in arXiv:1506.08386 [math.AG]. We also give a simple proof of the exactness of a sequence of motivic Galois groups under a Galois extension of the base field, which applies to all the above (Tannakian) situations. Finally, we clarify the construction of the categories of Chow-Lefschetz motives given in arXiv:2302.08327 [math.AG] and simplify the computation of their motivic Galois group in the numerical case.

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arXiv2001

Orbifold cohomology for global quotients

For an orbifold X which is the quotient of a manifold Y by a finite group G we construct a noncommutative ring with an action of G such that the orbifold cohomology of X as defined in math.AG/0004129 by Chen and Ruan is the G invariant part. In the case thar Y is S^n for a surface S with trivial canonical class we prove that (a small modification of) the orbifold cohomology of X is naturally isomorphic to the cohomology ring of the Hilbert scheme of n points on S computed in math.AG/0012166 by Lehn and Sorger.

math.AG↗
arXiv2001

Noncommutative smoothness and coadjoint orbits

Raf Bocklandt and the author have proved in math.AG/0010030 that certain quotient varieties of representations of deformed preprojective algebras are coadjoint orbits for the necklace Lie algebra of the corresponding quiver. A conjectural ringtheoretical explanation of these results was given in terms of noncommutative smoothness in the sense of C. Procesi. In this paper we prove these conjectures. The main tool in the proof is the etale local description due to W. Crawley-Boevey in math.AG/0105247. Along the way we determine the smooth locus of the so called Marsden-Weinstein reductions for quiver representations.

math.AG↗
arXiv2002

Deformation inequivalent complex conjugated complex structures and applications

Here, we resume and broaden the results concerned which appeared in math.AG/0101098 and math.AG/0104021. We start from summing up our example of a complex algebraic surface which is not deformation equivalent to its complex conjugate and which, moreover, has no homeomorphisms reversing the canonical class. Then, we construct several series of higher dimensional compact complex manifolds having the same property. We end with discussing applications to the Dif=Def problems, to the existence of diffeomorphic plane cuspidal curves non equivalent under equisingular deformations and to the existence of (deformation) non equivalent symplectic structures with opposite canonical classes.

math.AG↗
arXiv2003

Bounds for log canonical thresholds with applications to birational rigidity

We use intersection theory, degeneration techniques and jet schemes to study log canonical thresholds. Our first result gives a lower bound for the log canonical threshold of a pair in terms of the log canonical threshold of the image by a suitable smooth morphism. This in turn is based on an inequality relating the log canonical threshold and the Samuel multiplicity, generalizing our previous result from math.AG/0205171. We then give a lower bound for the log canonical threshold of an affine scheme defined by homogeneous equations of the same degree in terms of the dimension of the non log terminal locus (this part supersedes math.AG/0105113). As an application of our results, we prove the birational superrigidity of every smooth hypersurface of degree N in P^N, if 4\leq N\leq 12.

math.AG↗
arXiv2003

Simple singularities and integrable hierarchies

The paper math.AG/0108100 gives a construction of the total descendent potential corresponding to a semisimple Frobenius manifold. In math.AG/0209205, it is proved that the total descendent potential corresponding to K. Saito's Frobenius structure on the parameter space of the miniversal deformation of the A_{n-1}-singularity satisfies the modulo-n reduction of the KP-hierarchy. In this paper, we identify the hierarchy satisfied by the total descendent potential of a simple singularity of the A,D,E-type. Our description of the hierarchy is parallel to the vertex operator construction of Kac -- Wakimoto except that we give both some general integral formulas and explicit numerical values for certain coefficients which in the Kac -- Wakimoto theory are studied on a case-by-case basis and remain, generally speaking, unknown.

math.AG↗
arXiv2003

On Correspondences of a K3 Surface with itself I

Let X be a K3 surface with a polarization H with H^2=2rs. Assume that H.N(X)=Z for the Picard lattice N(X). The moduli space Y of sheaves over X with the Mukai vector (r,H,s) is again a K3 surface. We prove that Y\cong X, if there exists h_1\in N(X) with (h_1)^2=f(r,s), H.h_1\equiv 0\mod g(r,s), and h_1 satisfies some condition of primitivity. Existence of such type a criterion is surprising, and also gives some geometric interpretation of elements in N(X) with negative square. We describe all 18-dimensional irreducible components of moduli of the (X,H) with Y\cong X and prove that their number is infinite. These generalizes results of math.AG/0206158, math.AG/0304415 for r=s.

math.AG↗
arXiv2003

Appendix to "Welschinger invariant and enumeration of real rational curves"

The invariance of the Welschinger numbers for real unnodal Del Pezzo surfaces, which we used for the enumeration of real rational curves on real toric Del Pezzo surfaces (see math.AG/0303378 and IMRN 49 (2003), 2639-2653), follows from J.-Y. Welschinger's theorem stated and proved in a symplectic setting (math.AG/0303145 and CRAS, Ser. I, 336 (2003), 341-344). Here, we give an algebraic geometry version of the proof of the above invariance.

math.AG↗
arXiv2005

Configurations in abelian categories. II. Moduli stacks

This paper has been withdrawn, because I have merged it with paper I of the series, math.AG/0312190. The main results of this paper now appear in sections 7-9 of the revised version of math.AG/0312190, with shortened and improved proofs.

math.AG↗
arXiv2003

Alternating formulas for K-theoretic quiver polynomials

The main theorem here is the K-theoretic analogue of the cohomological `stable double component formula' for quiver functions in [Knutson, Miller, and Shimozono, math.AG/0308142]. This K-theoretic version is still in terms of lacing diagrams, but nonminimal diagrams contribute terms of higher degree. The motivating consequence is a conjecture of A. Buch on the sign-alternation of the coefficients appearing in his expansion of quiver K-polynomials in terms of stable Grothendieck polynomials for partitions [Buch, math.AG/0104029].

math.CO↗
arXiv2004

Rational curves on homogeneous cones

Let G/Q be an homogeneous variety embedded in a projective space P thanks to an ample line bundle L. Take a projective space containing P and form the cone X over G/Q, we call this a cone over an homogeneous variety. Let $α$ a class of 1-cycle on X. In this article we describe the irreducible components of the scheme of morphisms of class $α$ from a rational curve to X. The situation depends on the line bundle L : if the projectivised tangent space to the vertex contains lines (i.e. if G/Q contains lines in P) then the irreducible components are described as in our paper math.AG/0407123 by the difference between Cartier and Weil divisors. On the contrary if there is no line in the projectivised tangent space to the vertex then there are new irreducible components corresponding to the multiplicity of the curve through the vertex. As in math.AG/0407123 we use a resolution Y of X (the blowing-up) and study the curves on Y.

math.AG↗
arXiv2004

Frobenius-unstable bundles and p-curvature

We use the theory of p-curvature of connections to analyze stable vector bundles of rank 2 on curves of genus 2 which pull back to unstable bundles under the Frobenius morphism. We take two approaches, first using explicit formulas for p-curvature to analyze low-characteristic cases, and then using degeneration techniques to obtain an answer for a general curve by degenerating to an irreducible rational nodal curve, and applying the results of math.AG/0409145 and math.AG/0407445. We also apply our explicit formulas to give a new description of the strata of curves of genus 2 of different p-rank.

math.AG↗
arXiv2005

Invariants of real symplectic 4-manifolds out of reducible and cuspidal curves

We construct invariants under deformation of real symplectic 4-manifolds. These invariants are obtained by counting three different kinds of real rational J-holomorphic curves which realize a given homology class and pass through a given real configuration of (the appropriate number of) points. These curves are cuspidal curves, reducible curves and curves with a prescribed tangent line at some real point of the configuration. They are counted with respect to some sign defined by the parity of their number of isolated real double points and in the case of reducible curves, with respect to some mutiplicity. In the case of the complex projective plane equipped with its standard symplectic form and real structure, these invariants coincide with the ones previously constructed in math.AG/0303145. This leads to a relation between the count of real rational J-holomorphic curves done in math.AG/0303145 and the count of real rational reducible J-holomorphic curves presented here.

math.SG↗
arXiv2005

Groupe de Picard et groupe de Brauer des compactifications lisses d'espaces homog`enes

This is a thoroughly revised version of math.AG/0502516v1 (24 Feb. 2005). Let k be a field of characteristic zero. Let Y=G/H, where G is a connected linear algebraic group over k and H is a connected closed k-subgroup of G. Let X be a smooth compactification of Y over k. We prove the conjecture set forward in the previous version : the Galois-lattice given by the geometric Picard group of X is flasque. The previous version had only partial results in this direction. They were obtained at the expense of a long d'etour via local and global fields. We owe the drastic improvement to a suggestion by O. Gabber. The previous version also assumed G semisimple simply connected. We can dispense with this. The result now covers the previously known case Y=G, with G an arbitrary connected linear algebraic group. ----- Ceci est une version enti`erement r'evis'ee de math.AG/0502516v1 (24 f'ev. 2005). Soient k un corps de caract'eristique z'ero, G un k-groupe lin'eaire connexe et H un k-sous-groupe ferm'e connexe de G. Notons Y=G/H. Soit X une k-compactification lisse de Y. Dans la pr'ec'edente version, nous avancions la conjecture : le module galoisien donn'e par le groupe de Picard g'eom'etrique de X (c'est un r'eseau) est un module flasque. Nous 'etablissions des cas particuliers de cette conjecture, sous l'hypoth`ese suppl'ementaire que G est semi-simple simplement connexe, au moyen d'une r'eduction alambiqu'ee au cas des corps p-adiques. Le pr'esent texte, qui doit beaucoup à une suggestion d'O. Gabber, 'etablit la conjecture dans le cas g'en'eral.

math.AG↗
arXiv2005

A presentation for the Chow ring A^*(\bar{M}_{0,2}(P^1,2))

The purpose of this dissertation is to study the intersection theory of the moduli spaces of stable maps of degree two from two-pointed, genus zero nodal curves to arbitrary-dimensional projective space. Toward this end, first the Betti numbers of \bar{M}_{0,2}(P^r,2) are computed using Serre polynomials and equivariant Serre polynomials. Then, specializing to the space \bar{M}_{0,2}(P^1,2), generators and relations for the Chow ring are given. Chow rings of simpler spaces are also described, and the method of localization and linear algebra is developed. Both tools are used in finding the relations. It is further demonstrated that no additional relations exist among the generators, so that a presentation for the Chow ring A^*(\bar{M}_{0,2}(P^1,2)) is obtained. As a further check of the presentation, it is applied to give a new computation of the previously known genus zero, degree two, two-pointed gravitational correlators of P^1. Portions of this work also appear in math.AG/0501322 and math.AG/0504575, but the dissertation contains significantly more background and detail for those who may be interested in these. The dissertation is preserved in original form except for spacing changes, elimination of some front and end materials, additions to some references, and correction of typos in Proposition 11.

math.AG↗
arXiv2005

Numerical trivial foliations, Iitaka Fibrations and the numerical dimension

Modifying the notion of numerically trivial foliation of a pseudo-effective line bundle L introduced by the author in math.AG/0304312 it can be shown that the leaves of this foliation have codimension bigger or equal to the numerical dimension of L as defined by Boucksom, Demailly, Paun and Peternell, math.AG/0405285. Furthermore, if the Kodaira dimension of L equals its numerical dimension the Kodaira-Iitaka fibration is its numerically trivial foliation. Both statements together yield a sufficient criterion for L not being abundant.

math.AG↗
arXiv2006

Cluster X-varieties, amalgamation and Poisson-Lie groups

Starting from a split semisimple real Lie group G with trivial center, we define a family of varieties with additional structures. We describe them as the cluster X-varieties, as defined in math.AG/0311245. In particular they are Poisson varieties. We define Poisson maps of them to the group G with the standard Poisson-Lie structure. We introduce an operation of amalgamation of cluster varieties. Our varieties are amalgamations of elementary ones, assigned to positive simple roots of the root system of G. Some of them are very closely related to the double Bruhat cells. This paper is a building block in a description of the cluster structure of the moduli spaces of local systems on surfaces studied in math.AG/0311149.

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