Exact sequences for equivariantly formal spaces

Torus actions of complexity one in non-general position

Let the compact torus $T^{n-1}$ act on a smooth compact manifold $X^{2n}$ effectively with nonempty finite set of fixed points. We pose the question: what can be said about the orbit space $X^{2n}/T^{n-1}$ if the action is cohomologically equivariantly formal (which essentially means that $H^{odd}(X^{2n};\mathbb{Z})=0$). It happens that homology of the orbit space can be arbitrary in degrees $3$ and higher. For any finite simplicial complex $L$ we construct an equivariantly formal manifold $X^{2n}$ such that $X^{2n}/T^{n-1}$ is homotopy equivalent to $Σ^3L$. The constructed manifold $X^{2n}$ i…

Equivariant Intersection Cohomology of Toric Varieties

We investigate the equivariant intersection cohomology of a toric variety. Considering the defining fan of the variety as a finite topological space with the subfans being the open sets (that corresponds to the "toric" topology given by the invariant open subsets), equivariant intersection cohomology provides a sheaf (of graded modules over a sheaf of graded rings) on that "fan space". We prove that this sheaf is a "minimal extension sheaf", i.e., that it satisfies three relatively simple axioms which are known to characterize such a sheaf up to isomorphism. In the verification of the second o…

一类复杂度为一的等变形式 torus 作用的轨道空间

Orbit spaces of equivariantly formal torus actions of complexity one

本文研究了一类复杂度为一的 torus 作用的轨道空间，证明了其同调球面性质，并给出了等变形式的判据及 betti 数的表达式。

GKM sheaves and nonorientable surface group representations

Let T be a compact torus and X a nice compact T-space (say a manifold or variety). We introduce a functor assigning to X a "GKM-sheaf" F_X over a "GKM-hypergraph" G_X. Under the condition that X is equivariantly formal, the ring of global sections of F_X are identified with the equivariant cohomology, H_T^*(X; C). We show that GKM-sheaves provide a general framework able to incorporate numerous constructions in the GKM-theory literature. In the second half of the paper we apply these ideas to study the equivariant topology of the representation variety R_K := Hom(π_1(S), K) under conjugation b…

小圆盘操作元的等变形式性

Equivariant formality of the little disks operad

本文证明了小n-圆盘操作元在有理数上关于SO(n)和O(n)是等变形式的，等价于有向和无向框架小圆盘操作元作为∞-操作元是有理形式的。

On the equivariant cohomology of $\mathbb{Z}_2\times\mathbb{Z}_k$-symmetric spaces

$Γ$-symmetric spaces are a vast generalization of symmetric spaces. Previous results make it conceivable that their isotropy action is equivariantly formal, and we provide evidence for this in case that $Γ= \mathbb{Z}_2\times\mathbb{Z}_k$. This in particular implies that $\mathbb{Z}_2\times\mathbb{Z}_k$-symmetric spaces are formal in the sense of Rational Homotopy Theory.

The Toral Rank Conjecture and variants of equivariant formality

An action of a compact Lie group is called equivariantly formal, if the Leray--Serre spectral sequence of its Borel fibration degenerates at the E_2-term. This term is as prominent as it is restrictive. In this article, also motivated by the lack of junction between the notion of equivariant formality and the concept of formality of spaces (surging from rational homotopy theory) we suggest two new variations of equivariant formality: "MOD-formal actions" and "actions of formal core". We investigate and characterize these new terms in many different ways involving various tools from rational ho…

Positively curved GKM-manifolds

Let T be a torus of dimension at least k and M a T-manifold. M is a GKM_k-manifold if the action is equivariantly formal, has only isolated fixed points, and any k weights of the isotropy representation in the fixed points are linearly independent. In this paper we compute the cohomology rings with real and integer coefficients of GKM_3- and GKM_4-manifolds which admit invariant metrics of positive sectional curvature.

图为何不像流形？

How is a graph not like a manifold?

研究了等变形式作用下流形的GKM图性质，揭示了其与GKM流形的拓扑条件及上同调代数的关联。

Brieskorn球面是环面流形的轨道空间边界

Brieskorn spheres bound orbit spaces of torus manifolds

本文构造了边界为非平凡基本群Brieskorn同调3-球面的特定环面8-流形，并研究其向GKM图扩展的复杂度条件。

K-理论与形式性

K-theory and formality

本文计算了等变形式等迷作用的整系数等变K-理论，通过构造谱序列映射给出证明，并由此得出等变形式性的新刻画及弱等变形式性的等价条件。

Cohomology of GKM-sheaves

Let $T$ be a compact torus and $X$ be a a finite $T$-CW complex (e.g. a compact $T$-manifold). In earlier work, the second author introduced a functor which assigns to $X$ a so called GKM-sheaf $\mathcal{F}_X$ whose ring of global sections $H^0(\mathcal{F}_X)$ is isomorphic to the equivariant cohomology $H^*_T(X)$ whenever $X$ is equivariantly formal (meaning that $H^*_T(X)$ is a free module over $H^*(BT))$. In the current paper we prove more generally that $H^0(\mathcal{F}_X) \cong H^*_T(X)$ if and only if $H_T^*(X)$ is reflexive, and find a geometric interpretation of the higher cohomology $…

Landweber exactness of the formal group law in $c_1$-spherical bordism

We describe the structure of the coefficient ring $W^*(pt)=\varOmega_W^*$ of the $c_1$-spherical bordism theory for an arbitrary $SU$-bilinear multiplication. We prove that for any $SU$-bilinear multiplication the formal group of the theory $W^*$ is Landweber exact. Also we show that after inverting the set $\mathcal P$ of Fermat primes there exists a complex orientation of the localized theory $W^*[\mathcal P^{-1}]$ such that the coefficients of the corresponding formal group law generate the whole coefficient ring $\varOmega_W^*[\mathcal P^{-1}]$.

Comodule categories and the geometry of the stack of formal groups

We generalise recent results of M. Hovey and N. Strickland on comodule categories for Landweber exact algebras using the formalism of algebraic stacks.

Landweber exact formal group laws and smooth cohomology theories

The main aim of this paper is the construction of a smooth (sometimes called differential) extension \hat{MU} of the cohomology theory complex cobordism MU, using cycles for \hat{MU}(M) which are essentially proper maps W\to M with a fixed U(n)-structure and U(n)-connection on the (stable) normal bundle of W\to M. Crucial is that this model allows the construction of a product structure and of pushdown maps for this smooth extension of MU, which have all the expected properties. Moreover, we show, using the Landweber exact functor principle, that \hat{R}(M):=\hat{MU}(M)\otimes_{MU^*}R defines …