arXiv subjects

William Graham

Publications and source records attributed to William Graham.

29 records · Page 2Linked to original sources

arXiv2005

Nonabelian localization in equivariant K-theory and Riemann-Roch for quotients

We prove a localization formula in equivariant algebraic $K$-theory for an arbitrary complex algebraic group acting with finite stabilizer on a smooth algebraic space. This extends to non-diagonalizable groups the localization formulas H.A. Nielsen in equivariant $K$-theory of vector bundles and R.W. Thomason for higher $K$-theory of equivariant coherent sheaves. As an application we give a Riemann-Roch formula for quotients of smooth algebraic spaces by proper group actions. This formula extends previous work of B. Toen for stacks with quasi-projective moduli spaces and the authors for quotients by diagonalizable groups.

math.AG↗

arXiv2003

Nonemptiness of skew-symmetric degeneracy loci

Let V be a rank N vector bundle on a d-dimensional complex projective scheme X; assume that V is equipped with a skew-symmetric bilinear form with values in a line bundle L and that Λ^2 V^* \otimes L is ample. Suppose that the maximum rank of the form at any point of X is r, where r>0 is even. The main result of this paper is that if d>2(N-r), then the locus of points where the rank of the form is at most r-2 is nonempty. This is a skew-symmetric analogue of the main result of math.AG/0305159; the proof is similar. If the hypothesis of ampleness is relaxed, we obtain a weaker estimate on the maximum dimension of X (and give a similar result for the symmetric case). We give applications to subschemes of skew-symmetric matrices, and to the stratification of the dual of a Lie algebra by orbit dimension.

math.AG↗

arXiv2003

Nonemptiness of symmetric degeneracy loci

Let V be a rank N vector bundle on a d-dimensional complex projective scheme X; assume that V is equipped with a quadratic form with values in a line bundle L and that S^2 V^* \otimes L is ample. Suppose that the maximum rank of the quadratic form at any point of X is r > 0. The main result of this paper is that if d > N-r, then the locus of points where the rank of the quadratic form is at most r-1 is nonempty. We give some applications to subschemes of matrices, and to degeneracy loci associated to embeddings in projective space. The paper concludes with an appendix on Gysin maps. The main result of the appendix identifies a Gysin map with the natural map from ordinary to relative cohomology.

math.AG↗

arXiv2003

Riemann-Roch for quotients and Todd classes of simplicial toric varieties

In this paper we give an explicit formula for the Riemann-Roch map for singular schemes which are quotients of smooth schemes by diagonalizable groups. As an application we obtain a simple proof of a formula for the Todd class of a simplicial toric variety. An equivariant version of this formula was previously obtained for complete simplicial toric varieties by Brion and Vergne using different techniques.

math.AG↗

arXiv2000

Good representations and solvable groups

We give a characterization of connected solvable groups in terms of the existence of representations with certain geometric properties. The existence of such representations for the group of upper triangular matrices played an important role in the proof of the authors' equivariant Riemann-Roch theorem.

math.AG↗

arXiv1999

Riemann-Roch for equivariant Chow groups

The purpose of this paper is to prove an equivariant Riemann-Roch theorem for schemes or algebraic spaces with an action of a linear algebraic group $G$. For a $G$-space $X$, this theorem gives an isomorphism between a completion of the equivariant Grothendieck group and a completion of equivariant equivariant Chow groups. The key to proving this isomorphism is a geometric description of completions of the equivariant Grothendieck group. Besides Riemann-Roch, this result has some purely $K$-theoretic applications. In particular, we prove a conjecture of Köck (in the case of regular schemes) and extend to arbitrary characteristic a result of Segal on representation rings.

math.AG↗

arXiv1999

Positivity in equivariant Schubert calculus

We prove a conjecture of Dale Peterson on positivity in the multiplication in the T-equivariant cohomology of the flag variety. The theorem follows from a more general positivity result about the equivariant cohomology of varieties with actions of a solvable group with finitely many orbits. This more general result is an equivariant version of a theorem of Kumar and Nori.

math.AG↗

arXiv1997

Equivariant intersection theory

This is a revised and shortened version of our paper "Equivariant intersection theory" (alg-geom/9603008). In particular, the sections on Riemann-Roch and localization are omitted. They will appear in separate papers, at which time alg-geom/9603008 will become obsolete. We have intsead added a section of examples, and have include a calculation of the integral Chow ring of the mdouli stack of elliptic curves.

alg-geom↗

arXiv1996

Localization in equivariant intersection theory and the Bott residue formula

The purpose of this paper is to prove the localization theorem for torus actions in equivariant intersection theory. Using the theorem we give another proof of the Bott residue formula for Chern numbers of bundles on smooth complete varieties. In addition, our techniques allow us to obtain residue formulas for bundles on a certain class of singular schemes which admit torus actions. This class is rather special, but it includes some interesting examples such as complete intersections and Schubert varieties.

alg-geom↗

arXiv1996

Algebraic Cuts

Let $X$ be a projective variety with a torus action, which for simplicity we assume to have dimension 1. If $X$ is a smooth complex variety, then the geometric invariant theory quotient $X//G$ can be identifed with the symplectic reduction $X_r$. Lerman introduced a construction (valid for symplectic manifolds) called symplectic cutting, which constructs a manifold $X_c$, such that $X_c$ is the union of $X_r$ and an open subset $X_{>0} \subset X$. Moreover, there is a natural torus action on $X_c$ such that $X_r$ is a component of the fixed locus. Using localization for equivariant cohomology, this construction can be used to study of $X_r$. In this note, we give an algebraic version of this construction valid for projective but possibly singular varieties defined over arbitrary fields. This construction is useful for studying $X_r$ from the point of view of algebraic geometry, using the equivariant intersection theory developed by the authors. At the end of the paper we briefly give an adaptation of Lerman's proof of the Kalkman residue formula and use it to give some formulas for characteristic numbers of quotients by a torus.

alg-geom↗

arXiv1996

Equivariant intersection theory

In this paper we develop an equivariant intersection theory for actions of algebraic groups on algebraic schemes. The theory is based on our construction of equivariant Chow groups. They are algebraic analogues of equivariant cohomology groups which satsify the formal properties of ordinary Chow groups. In addition, they enjoy many of the properties of equivariant cohomology. The principal results are: (1) We prove the existence of canonical intersection products on the Chow groups of geometric quotients of smooth varieties- even when the stabilizers of geometric points are non-reduced. (2) We construct a Todd class map from equivariant $K$-theory of coherent sheaves to a completion of equivariant Chow groups, and prove that a completion of equivariant $K$-theory is isomorphic to the completion of equivariant Chow groups. (3) We prove a localization theorem for torus actions and use it to give a characteristic free proof of the Bott residue formula for actions of tori on complete smooth varieties.

alg-geom↗