arXiv subjects

Lisa C. Jeffrey

Publications and source records attributed to Lisa C. Jeffrey.

27 records · Page 2Linked to original sources

arXiv2002

Products of conjugacy classes in SU(2)

We obtain a complete description of collections of n conjugacy classes in SU(2) with the property that the multiplication map from the product of these n conjugacy classes to SU(2) is surjective. The basic instrument is a characterization of collections of n+1 conjugacy classes in SU(2) such that the image of the multiplication map from these conjugacy classes to SU(2) contains the identity matrix, which generalizes a result of Jeffrey and Weitsman.

math.GR↗

arXiv2002

The Kernel of the Equivariant Kirwan Map and the Residue Formula

Using the notion of equivariant Kirwan map, as defined by Goldin, we prove that -- in the case of Hamiltonian torus actions with isolated fixed points -- Tolman and Weitsman's description of the kernel of the Kirwan map can be deduced directly from the residue theorem of Jeffrey and Kirwan. A characterization of the kernel of the Kirwan map in terms of residues of one variable (i.e. associated to Hamiltonian circle actions) is obtained.

math.SG↗

arXiv2002

The residue formula and the Tolman-Weitsman theorem

We give a simple direct proof (for the case of Hamiltonian circle actions with isolated fixed points) that Tolman and Weitsman's description of the kernel of the Kirwan map (in other words the sum of those equivariant cohomology classes vanishing on one side of a collection of hyperplanes) is equivalent to the characterization of this kernel given by the residue theorem of Jeffrey and Kirwan.

math.SG↗

arXiv2000

The Verlinde formula for parabolic bundles

The moduli space M(n,d) is an algebraic variety parametrizing those representations of the fundamental group of a punctured Riemann surface into the Lie group SU(n) for which a loop around the boundary is sent to the n-th root of unity exp (2 πi d/n) multiplied by the identity matrix. If n and d are coprime it is in fact a Kähler manifold. One may relax the constraint and study moduli spaces M(L) parametrizing those representations for which the loop around the boundary is sent to an element conjugate to L, if L is some element in SU(n), and these are also Kähler manifolds for a suitable class of L. The Verlinde formula calculates the dimension of the space of holomorphic sections of certain line bundles over the spaces M(n,d) and M(L). We recall how a new proof of the Verlinde formula for M(n,d) given in joint work with Frances Kirwan may be obtained, and show how to modify this proof to obtain a proof of the variant of the Verlinde formula which applies to M(L) (the moduli space of parabolic bundles with one marked point) and to the analogous object with b marked points.

math.AG↗

arXiv1999

Representations with Weighted Frames and Framed Parabolic Bundles

There is a well-known correspondence between the symplectic variety of representations of the fundamental group of a punctured Riemann surface into a compact Lie group G, with fixed conjugacy classes at the punctures, and a complex variety of holomorphic bundles on the unpunctured surface with a parabolic structure at the puncture points. For G=SU(2), we build a symplectic variety of pairs (representations of the fundamental group into G, `weighted frame' at the puncture points), and a corresponding complex variety of moduli of `framed parabolic bundles', which encompass respectively all of the above spaces, in the sense that one can obtain the former from the latter by taking a symplectic quotient or a geometric invariant theory quotient. This allows us to explain certain features of the toric geometry of the SU(2) moduli spaces discussed by Jeffrey and Weitsman, by giving the actual toric variety associated with their integrable system.

math.SG↗

arXiv1997

Intersection theory on moduli spaces of holomorphic bundles of arbitrary rank on a Riemann surface

We prove formulas (found by Witten in 1992 using physical methods) for intersection pairings in the cohomology of the moduli space M(n,d) of stable holomorphic vector bundles of rank n and degree d (assumed coprime) on a Riemann surface of genus g greater than or equal to 2. We also use these formulas for intersection numbers to obtain a proof of the Verlinde formula for the dimension of the space of holomorphic sections of a line bundle over M(n,d).

alg-geom↗

arXiv1996

Symplectic Forms on Moduli Spaces of Flat Connections on 2-manifolds

Let $G$ be a compact connected semisimple Lie group. We extend the techniques of Weinstein [W] to give a construction in group cohomology of symplectic forms $ω$ on \lq twisted' moduli spaces of representations of the fundamental group $π$ of a 2-manifold $Σ$ (the smooth analogues of ${\rm Hom} (π_1(Σ), G)/G$) and on relative character varieties of fundamental groups of 2-manifolds. We extend this construction to exhibit a symplectic form on the extended moduli space [J1] (a Hamiltonian $G$-space from which these moduli spaces may be obtained by symplectic reduction), and compute the moment map for the action of $G$ on the extended moduli space.

alg-geom↗

arXiv1995

On localization and Riemann-Roch numbers for symplectic quotients

Suppose $(M,ω)$ is a compact symplectic manifold acted on by a compact Lie group $K$ in a Hamiltonian fashion, with moment map $μ: M \to \Lie(K)^*$ and Marsden-Weinstein reduction $M_{red} = μ^{-1}(0)/K$. In this paper, we assume that $M$ has a $K$-invariant Kähler structure. In an earlier paper, we proved a formula (the residue formula) for $η_0 e^{ω_0}[M_{red}]$ for any $η_0 \in H^*(M_{red})$, where $ω_0$ is the induced symplectic form on $M_{red}$. Here we apply the residue formula in the special case $η_0 = Td(M_{red})$; when $K$ acts freely on $μ^{-1}(0)$ this yields a formula for the Riemann-Roch number $RR (L_{red})$ of a holomorphic line bundle $L_{red}$ on $M_{red}$ that descends from a holomorphic line bundle $L$ on $M$ for which $c_1(L) = ω$. Using the holomorphic Lefschetz formula we similarly obtain a formula for the $K$-invariant Riemann-Roch number $RR^K(L) $ of $L$. In the case when the maximal torus $T$ of $K$ has dimension one (except in a few special circumstances), we show the two formulas are the same. Thus in this special case the residue formula is equivalent to the result of Guillemin and Sternberg that $RR(L_{red}) = RR^K(L)$. (The residue formula was proved under the assumption that 0 is a regular value of $μ$, and was given in terms of the restrictions of classes in the equivariant cohomology $H^*_T(M) $ of $M$ to the

alg-geom↗

arXiv1994

Group cohomology construction of the cohomology of moduli spaces of flat connections on 2-manifolds

We use group cohomology and the de Rham complex on simplicial manifolds to give explicit differential forms representing generators of the cohomology rings of moduli spaces of representations of fundamental groups of 2-manifolds. These generators are constructed using the de Rham representatives for the cohomology of classifying spaces $BK$ where $K$ is a compact Lie group; such representatives (universal characteristic classes) were found by Bott and Shulman. Thus our representatives for the generators of the cohomology of moduli spaces are given explicitly in terms of the Maurer-Cartan form. This work solves a problem posed by Weinstein, who gave a corresponding construction (following Karshon and Goldman) of the symplectic forms on these moduli spaces. We also give a corresponding construction of equivariant differential forms on the extended moduli space $X$, which is a finite dimensional symplectic space equipped with a Hamiltonian action of $K$ for which the symplectic reduced space is the moduli space of representations of the 2-manifold fundamental group in $K$. (This paper is in press in Duke Math. J. The substance of the text is unaltered; inor changes and corrections have been made to the file to correspond to the version that will be published.)

alg-geom↗