The Duistermaat-Heckman formula and Chern-Schwartz-MacPherson classes

The log-concavity conjecture for the Duistermaat-Heckman measure revisited

Karshon constructed the first counterexample to the log-concavity conjecture for the Duistermaat-Heckman measure: a Hamiltonian six manifold whose fixed points set is the disjoint union of two copies of $T^4$. In this article, for any closed symplectic four manifold $N$ with $b+$ greater than 1, we show that there is a Hamiltonian six manifold $M$ such that its fixed points set is the disjoint union of two copies of $N$ and such that its Duistermaat-Heckman function is not log-concave. On the other hand, we prove that if there is a torus action of complexity two such that all the symplectic re…

The Duistermaat-Heckman formula and the cohomology of moduli spaces of polygons

We give a presentation of the cohomology ring of spatial polygon spaces $M(r)$ with fixed side lengths $r \in \mathbb R^n_+$. These spaces can be described as the symplectic reduction of the Grassmaniann of 2-planes in $\mathbb C^n$ by the $U(1)^n$-action by multiplication, where $U(1)^n$ is the torus of diagonal matrices in the unitary group U(n). We prove that the first Chern classes of the $n$ line bundles associated with the fibration $r$-level set $\rightarrow M(r)$ generate the cohomology ring $H^* (M(r), \mathbb C).$ By applying the Duistermaat--Heckman Theorem, we then deduce the relat…

杜伊斯特马-海克曼测度在量子信息理论中的一个应用

One application of Duistermaat-Heckman measure in quantum information theory

本文通过杜伊斯特马-海克曼测度推导了两量子比特状态下分离概率为8/33的理论结果，结合辛几何与表示论的方法，提供了透明的推导路径。

Characters, Coadjoint Orbits and Duistermaat-Heckman Integrals

The asymptotics of characters $χ_{kλ}(\exp(h/k))$ of irreducible representations of a compact Lie group $G$ for large values of the scaling factor $k$ are given by Duistermaat-Heckman (DH) integrals over coadjoint orbits of $G$. This phenomenon generalises to coadjoint orbits of central extensions of loop groups $\widehat{LG}$ and of diffeomorphisms of the circle $\widehat{\rm Diff}(S^1)$. We show that the asymptotics of characters of integrable modules of affine Kac-Moody algebras and of the Virasoro algebra factorize into a divergent contribution of the standard form and a convergent contrib…

A vector partition function for the multiplicities of sl_k(C)

We use Gelfand-Tsetlin diagrams to write down the weight multiplicity function for the Lie algebra sl_k(C) (type A_{k-1}) as a single partition function. This allows us to apply known results about partition functions to derive interesting properties of the weight diagrams. We relate this description to that of the Duistermaat-Heckman measure from symplectic geometry, which gives a large-scale limit way to look at multiplicity diagrams. We also provide an explanation for why the weight polynomials in the boundary regions of the weight diagrams exhibit a number of linear factors. Using symplect…

The mod 2 cohomology of fixed point sets of anti-symplectic involutions

Let $M$ be a compact, connected symplectic manifold with a Hamiltonian action of a compact $n$-dimensional torus $G=T^n$. Suppose that $σ$ is an anti-symplectic involution compatible with the $G$-action. The real locus of $M$ is $X$, the fixed point set of $σ$. Duistermaat uses Morse theory to give a description of the ordinary cohomology of $X$ in terms of the cohomology of $M$. There is a residual $\G=(\Zt)^n$ action on $X$, and we can use Duistermaat's result, as well as some general facts about equivariant cohomology, to prove an equivariant analogue to Duistermaat's theorem. In some cases…

Norm-square localization for Hamiltonian $LG$-spaces

We prove a formula for twisted Duistermaat-Heckman distributions associated to a Hamiltonian $LG$-space. The terms of the formula are localized at the critical points of the norm-square of the moment map, and can be computed in cross-sections. Our main tools are the theory of quasi-Hamiltonian $G$-spaces, as well as the Hamiltonian cobordism approach to norm-square localization introduced recently by Harada and Karshon.

叠哈密顿作用与辛约化

Stacky Hamiltonian actions and symplectic reduction

本文引入了étale李群叠在étale辛叠上的哈密顿作用概念，并在该背景下证明了Kirwan凸性定理、Meyer-Marsden-Weinstein辛约化定理及Duistermaat-Heckman定理的版本。

Jump formulas in Hamiltonian Geometry

This paper is concerned with the Hamiltonian actions of a torus on a symplectic manifold. We are interested here in two global invariants: the Duistermaat-Heckman measure DH(M), and the Riemann-Roch chatacters RR(M,L^k),k>0, which are defined when the symplectic manifold is prequantized by a Kostant-Souriau line bundle L. We can associate to each connected component C of regular values of the moment map the following local invariants: the polynomial DH_c which coincides with DH(M) on C, and the periodic polynomial m_c which computes the multiplicity of RR(M,L^k), k>0, in the cone generated by …

余辛几何中的哈密顿群作用

Hamiltonian group actions in cosymplectic geometry

研究余辛流形上哈密顿群作用，通过辛加厚过程，将其提升为辛流形上哈密顿辛作用，建立相关理论桥梁，得出凸性定理等余辛类似结果，还应用于b-辛流形临界超曲面的哈密顿几何。

The symplectic geometry of closed equilateral random walks in 3-space

A closed equilateral random walk in 3-space is a selection of unit length vectors giving the steps of the walk conditioned on the assumption that the sum of the vectors is zero. The sample space of such walks with $n$ edges is the $(2n-3)$-dimensional Riemannian manifold of equilateral closed polygons in $\mathbb{R}^3$. We study closed random walks using the symplectic geometry of the $(2n-6)$-dimensional quotient of the manifold of polygons by the action of the rotation group $\operatorname {SO}(3)$. The basic objects of study are the moment maps on equilateral random polygon space given by t…