On homology classes not representable by products

Representing homology classes by symplectic surfaces

We derive an obstruction to representing a homology class of a symplectic 4-manifold by an embedded, possibly disconnected, symplectic surface.

Homology classes of negative square and embedded surfaces in 4-manifolds

Let X be a simply-connected closed oriented 4-manifold and A an embedded surface of genus g and negative self-intersection -N. We show that for fixed genus g there is an upper bound on N if the homology class of A is divisible or characteristic. In particular, for genus zero, there is a lower bound on the self-intersection of embedded spheres in these kinds of homology classes. This question is related to a problem from the Kirby list.

Geometric construction of homology classes in Riemannian manifolds covered by products of hyperbolic planes

We study the homology of Riemannian manifolds of finite volume that are covered by an $r$-fold product $(\mathbb{H}^2)^r = \mathbb{H}^2 \times \ldots \times \mathbb{H}^2$ of hyperbolic planes. Using a variation of a method developed by Avramidi and Nguyen-Phan, we show that any such manifold $M$ possesses, up to finite coverings, an arbitrarily large number of compact oriented flat totally geodesic $r$-dimensional submanifolds whose fundamental classes are linearly independent in the homology group $H_r(M;\mathbb{Z})$.

The $E_8$-boundings of homology spheres and negative sphere classes in $E(1)$

We define invariants $\frak{ds}$ and $\overline{\frak{ds}}$, which are the maximal and minimal second Betti number divided by $8$ among definite spin boundings of a homology sphere. The similar invariants $g_8$ and $\overline{g_8}$ are defined by the maximal (or minimal) product sum of $E_8$-form of bounding 4-manifolds. We compute these invariants for some homology spheres. We construct $E_8$-boundings for some of Brieskorn 3-spheres $Σ(2,3,12n+5)$ by handle decomposition. As a by-product of the construction, some negative classes which consist of addition of several fiber classes plus one se…

纽结与正割类的多交叉复形

Multicrossing complex of knot and secant classes

该研究将纽结的多交叉同调推广自其基本 quandles 同调，证明纽结正割的部分和可在该同调中定义不变类。

The Minimal Genus of Homology Classes in a Finite 2-Complex

We study surface representatives of homology classes of finite complexes which minimize certain complexity measures, including its genus and Euler characteristic. Our main result is that up to surgery at nullhomotopic curves minimizers are homotopic to cellwise coverings to the 2-skeleton. From this we conclude that the minimizing problem is in general algorithmically undecidable, but can be solved for 2-dimensional CAT(-1)-complexes.

同调类的莫尔斯复杂度

Morse complexity of homology classes

研究流形、配边和同调类的莫尔斯复杂度，通过手术理论证上界、指标理论证下界，得出允许离散序列表示的李群的局部对称空间莫尔斯复杂度随体积线性增长，且不允许开书分解的结论。

On Kontsevich's characteristic classes for smooth 5- and 7-dimensional homology sphere bundles

M. Kontsevich constructed universal characteristic classes of smooth bundles with fiber a framed odd-dimensional integral homology sphere. In dimension 3, they are known to give a universal finite type invariants of homology 3-spheres. However, they have not been well understood for higher fiber dimensions. The purpose of the present paper is twofold. First, we obtain a bordism invariant of smooth unframed bundles with fiber a 5-dimensional homology sphere, which is defined as a sum of the simplest Kontsevich class and the second signature defect. It may be in some sense a higher dimensional a…

Khovanov-Rozansky同调中的平凡类与对称性

Tautological classes and symmetry in Khovanov-Rozansky homology

该研究在Khovanov-Rozansky链环同调中定义新交换算子族$F_k$，证明$F_2$满足硬莱夫谢茨性质，验证了相关学者提出的同调对称性猜想。

An absolute grading on Heegaard Floer homology by homotopy classes of oriented 2-plane fields

For a closed oriented 3-manifold Y, we define an absolute grading on the Heegaard Floer homology groups of Y by homotopy classes of oriented 2-plane fields. We show that this absolute grading refines the relative one and that it is compatible with the maps induced by cobordisms. We also prove that if ξ is a contact structure on Y, then the grading of the contact invariant c(ξ) is the homotopy class of ξ.

实现预定同调类的流形的显式构造

Explicit construction of manifolds realizing the prescribed homology classes

针对经典的 N. Steenrod 同调类实现问题，提出一种基于解决伪流形奇点局部程序的显式纯组合构造方法，为给定整循环提供实现其同调类倍数的组合流形，并应用于构造具有预定顶点链接集合的组合流形。

Grid homology and the unknotting number

We showed that the order of torsion homology classes in the grid homology of a knot is a lower bound for the unknotting number.

同调类中模子格的闭测地线

Closed geodesics in homology classes modulo sublattices

研究Weil-Petersson随机双曲曲面中同调类模素指数格的本原闭测地线分布，计算加权计数函数中心矩、分析方差渐近行为，结果类同算术级数素数分布现象。

大亏格随机双曲曲面上同调类中的闭测地线

Closed geodesics in homology classes on random hyperbolic surfaces of large genus

研究大亏格随机双曲曲面上同调类中闭测地线分布，通过将曲面视为模空间随机点，研究闭测地线加权计数函数波动，得出大亏格极限下不同模的方差情况，与胡利猜想对比并给出差异解释。

Unoriented topological quantum field theory and link homology

We investigate link homology theories for stable equivalence classes of link diagrams on orientable surfaces. We apply (1+1)-dimensional unoriented topological quantum field theories to Bar-Natan's geometric formalism to define new theories for stable equivalence classes.

Moving homology classes in finite covers of graphs

Let $Y\to X$ be a finite normal cover of a wedge of $n\geq 3$ circles. We prove that for any $v\neq 0\in H_1(Y;\mathbb{Q})$ there exists a lift $\widetilde{F}$ to $Y$ of a homotopy equivalence $F:X\to X$ so that the set of iterates $\{\widetilde{F}^d(v): d\in \mathbb{Z}\}\subseteq H_1(Y;\mathbb{Q})$ is infinite. The main achievement of this paper is the use of representation theory to prove the existence of a purely topological object that seems to be inaccessible via topology.

Homological eigenvalues of mapping classes and torsion homology growth for fibered 3--manifolds

Let S be an orientable surface with negative Euler characteristic, let ψ\in\Mod(S) be a mapping class of S, and let T_ψ be the mapping torus of ψ. We study the action of lifts of ψon the homology of finite covers of S via the torsion homology growth of towers of finite covers of T_ψ. We show that ψadmits a lift to a finite cover with a homological eigenvalue of length greater than one if and only if the mapping torus T_ψ admits a finite cover X and a certain tower of abelian covers which have exponential torsion homology growth. We show that the existence of such a lift of ψis intrinsic to T_ψ…

Perturbed Floer Homology of some Fibered Three Manifolds II

Modifying the method of [21], we compute the perturbed $HF^+$ for some special classes of fibered three manifolds in the second highest spin$^c$-structures $S_{g-2}$. The special classes considered in this paper include the mapping tori of Dehn twists along a single non-separating curve and along a transverse pair of curves.

Fibered Cohomology Classes in Dimension Three, Twisted Alexander Polynomials and Novikov Homology

We prove that for "most" closed 3-dimensional manifolds $M$, the existence of a closed non singular one-form in a given cohomology class $u\in H^1 (M,\bf R)$ is equivalent to the fact that every twisted Alexander polynomial $Δ^H(M,u) \in {\bf Z}[G/\ker u]$ associated to a normal subgroup with finite index $H < π_1(M)$ has a unitary $u$-minimal term.

Guts in Sutured Decompositions and the Thurston Norm

We construct an invariant called guts for second homology classes in irreducible 3-manifolds with toral boundary and non-degenerate Thurston norm. We prove that the guts of second homology classes in each Thurston cone are invariant under a natural condition. We show that the guts of different homology classes are related by sutured decompositions. As an application, an invariant of knot complements is given and is computed in a few interesting cases. Besides, we show that the dimension of a maximal simplex in a Kakimizu Complex is an invariant.

关于3-辫的Khovanov同调

On the Khovanov homology of 3-braids

本研究证明了3股辫闭包的Khovanov同调仅含2阶挠元的Przytycki-Sazdanovic猜想，补全了3-辫Murasugi分类中剩余一类的证明，同时验证了Knight移动猜想对3-辫成立。

Pseudo-Anosov stretch factors and homology of mapping tori

We consider the pseudo-Anosov elements of the mapping class group of a surface of genus g that fix a rank k subgroup of the first homology of the surface. We show that the smallest entropy among these is comparable to (k+1)/g. This interpolates between results of Penner and of Farb and the second and third authors, who treated the cases of k=0 and k=2g, respectively, and answers a question of Ellenberg. We also show that the number of conjugacy classes of pseudo-Anosov mapping classes as above grows (as a function of g) like a polynomial of degree k.

Comparing homological invariants for mapping classes of surfaces

We compare two different types of mapping class invariants: the Hochschild homology of an $A_\infty$ bimodule coming from bordered Heegaard Floer homology, and fixed point Floer cohomology. We first compute the bimodule invariants and their Hochschild homology in the genus two case. We then compare the resulting computations to fixed point Floer cohomology, and make a conjecture that the two invariants are isomorphic. We also discuss a construction of a map potentially giving the isomorphism. It comes as an open-closed map in the context of a surface being viewed as a $0$-dimensional Lefschetz…

The Bar-Natan homology and unknotting number

We show that the order of torsion homology classes in Bar-Natan deformation of Khovanov homology is a lower bound for the unknotting number. We give examples of knots that this is a better lower bound than |s(K)/2|, where s(K) is the Rasmussen s invariant defined by the Bar-Natan spectral sequence.

Morita classes in the homology of automorphism groups of free groups

Using Kontsevich's identification of the homology of the Lie algebra l_infty with the cohomology of Out(F_r), Morita defined a sequence of 4k-dimensional classes mu_k in the unstable rational homology of Out(F_{2k+2}). He showed by a computer calculation that the first of these is non-trivial, so coincides with the unique non-trivial rational homology class for Out(F_4). Using the "forested graph complex" introduced in [Algebr. Geom. Topol. 3 (2003) 1167--1224], we reinterpret and generalize Morita's cycles, obtaining an unstable cycle for every connected odd-valent graph. (Morita has independ…

Normal surfaces and colored Khovanov homology

We show that colored Khovanov homology detects classes of essential surfaces as a direct analogue of the slope conjectures for the colored Jones polynomial. We do this by identifying certain generators of the colored Khovanov chain complex with normal surfaces in the complement of the knot using an ideal triangulation from a diagram.

论 $\mathbb{L}$-同调类的几何表示

On geometric representation of $\mathbb{L}$-homology classes

本文建立法配边类与 $\mathbb{L}$-同调之间的关系，证明对 $n\ge6$ 及有限复形 $B$ 存在映射 $Γ$，从而以带参考映射的法度一映射几何表示 $\mathbb{L}$-同调类。

Computations in formal symplectic geometry and characteristic classes of moduli spaces

We make explicit computations in the formal symplectic geometry of Kontsevich and determine the Euler characteristics of the three cases, namely commutative, Lie and associative ones, up to certain weights.From these, we obtain some non-triviality results in each case. In particular, we determine the integral Euler characteristics of the outer automorphism groups Out F_n of free groups for all n <= 10 and prove the existence of plenty of rational cohomology classes of odd degrees. We also clarify the relationship of the commutative graph homology with finite type invariants of homology 3-spher…

On minimal actions of finite simple groups on homology spheres and Euclidean spaces

We consider the following problem: for which classes of finite groups, and in particular finite simple groups, does the minimal dimension of a faithful, smooth action on a homology sphere coincide with the minimal dimension of a faithful, linear action on a sphere? We prove that the two minimal dimensions coincide for the linear fractional groups PSL(2,p) as well as for various classes of alternating and symmetric groups. We prove analogous results also for actions on Euclidean spaces.

Higher hairy graph homology

We study the hairy graph homology of a cyclic operad; in particular we show how to assemble corresponding hairy graph cohomology classes to form cocycles for ordinary graph homology, as defined by Kontsevich. We identify the part of hairy graph homology coming from graphs with cyclic fundamental group as the dihedral homology of a related associative algebra with involution. For the operads Comm, Assoc and Lie we compute this algebra explicitly, enabling us to apply known results on dihedral homology to the computation of hairy graph homology. In addition we determine the image in hairy graph …

The action of mapping classes on nilpotent covers of surfaces

Let $Σ$ be a surface whose interior admits a hyperbolic structure of finite volume. In this paper, we show that any infinite order mapping class acts with infinite order on the homology of some universal $k$--step nilpotent cover of $Σ$. We show that a Torelli mapping class either acts with infinite order on the homology of a finite abelian cover, or the suspension of the mapping class is a 3--manifold whose fundamental group has positive homology gradient. In the latter case, it follows that the suspended 3--manifold has a large fundamental group. It follows that every element of the Magnus k…

从光滑4流形边界的链接同调论中表面不变量

Invariants of surfaces in smooth 4-manifolds from link homology

本文提出了一种基于变形$\mathfrak{gl}_N$链接同调的skein lasagna模块，用于构建光滑4流形边界的链接不变量，并建立了其分解定理和泛函性扩展方法。

Pseudo-Anosov braids with small entropy and the magic 3-manifold

We consider a hyperbolic surface bundle over the circle with the smallest known volume among hyperbolic manifolds having 3 cusps, so called "the magic manifold". We compute the entropy function on the fiber face of the unit ball with respect to the Thurston norm, determine homology classes whose representatives are genus 0 fiber surfaces, and describe their monodromies by braids. Among such homology classes whose representatives have n punctures, we decide which one realizes the minimal entropy. It turns out that all the braids with smallest known entropy are derived from monodromies for such …

(n,n+1) torus knots的tautological类

Tautological classes for (n,n+1) torus knots

本文研究了(n,n+1) torus knots的tautological类，并建立了其与HOMFLY-PT同调及对角不变量空间的联系，同时探讨了其在李代数和谱序列中的作用。