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Parabolic bundles, products of conjugacy classes, and Gromov-Witten invariants

Constantin Teleman, Christopher Woodward (2003)

Annales de l’institut Fourier

The set of conjugacy classes appearing in a product of conjugacy classes in a compact, 1 -connected Lie group K can be identified with a convex polytope in the Weyl alcove. In this paper we identify linear inequalities defining this polytope. Each inequality corresponds to a non-vanishing Gromov-Witten invariant for a generalized flag variety G / P , where G is the complexification of K and P is a maximal parabolic subgroup. This generalizes the results for S U ( n ) of Agnihotri and the second author and Belkale on...

Perfect stratifications and theory of weights.

Vicente Navarro Aznar (1992)

Publicacions Matemàtiques

In this paper we emphasize Deligne's theory of weights, in order to prove that some stratifications of algebraic varieties are perfect. In particular, we study in some detail the Bialynicki-Birula's stratifications and the stratifications considered by F. Kirwan to compute the cohomology of symplectic or geometric quotients. Finally we also appoint the motivic formulation of this approach, which contains the Hodge theoretic formulation.

Plane curves with small linear orbits, I

Paoli Aluffi, Carel Faber (2000)

Annales de l'institut Fourier

The “linear orbit” of a plane curve of degree d is its orbit in ℙ d ( d + 3 ) / 2 under the natural action of PGL ( 3 ) . In this paper we compute the degree of the closure of the linear orbits of most curves with positive dimensional stabilizers. Our tool is a nonsingular variety dominating the orbit closure, which we construct by a blow-up sequence mirroring the sequence yielding an embedded resolution of the curve. The results given here will serve as an ingredient in the computation of the analogous information for...

Predegree Polynomials of Plane Configurations in Projective Space

Tzigantchev, Dimitre (2008)

Serdica Mathematical Journal

2000 Mathematics Subject Classification: 14N10, 14C17.We work over an algebraically closed field of characteristic zero. The group PGL(4) acts naturally on PN which parameterizes surfaces of a given degree in P3. The orbit of a surface under this action is the image of a rational map PGL(4) ⊂ P15→PN. The closure of the orbit is a natural and interesting object to study. Its predegree is defined as the degree of the orbit closure multiplied by the degree of the above map restricted to a general Pj,...

Pseudo-abelian varieties

Burt Totaro (2013)

Annales scientifiques de l'École Normale Supérieure

Chevalley’s theorem states that every smooth connected algebraic group over a perfect field is an extension of an abelian variety by a smooth connected affine group. That fails when the base field is not perfect. We define a pseudo-abelian variety over an arbitrary field k to be a smooth connected k -group in which every smooth connected affine normal k -subgroup is trivial. This gives a new point of view on the classification of algebraic groups: every smooth connected group over a field is an extension...

Purity of level m stratifications

Marc-Hubert Nicole, Adrian Vasiu, Torsten Wedhorn (2010)

Annales scientifiques de l'École Normale Supérieure

Let k be a field of characteristic p > 0 . Let D m be a BT m over k (i.e., an m -truncated Barsotti–Tate group over k ). Let S be a k -scheme and let X be a BT m over S . Let S D m ( X ) be the subscheme of S which describes the locus where X is locally for the fppf topology isomorphic to D m . If p ≥ 5 , we show that S D m ( X ) is pure in S , i.e. the immersion S D m ( X ) ↪ S is affine. For p ∈ { 2 , 3 } , we prove purity if D m satisfies a certain technical property depending only on its p -torsion D m [ p ] . For p ≥ 5 , we apply the developed techniques to show that all level m ...

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