Virtual betti numbers of symmetric spaces

Sutured manifolds and $L^2$-Betti numbers

Using the virtual fibering theorem of Agol we show that a sutured 3-manifold $(M, R_+,R_-,γ)$ is taut if and only if the $\ell^2$-Betti numbers of the pair $(M,R_-)$ are zero. As an application we can characterize Thurston norm minimizing surfaces in a 3-manifold $N$ with empty or toroidal boundary by the vanishing of certain $\ell^2$-Betti numbers.

On the growth of Betti numbers of locally symmetric spaces

We announce new results concerning the asymptotic behavior of the Betti numbers of higher rank locally symmetric spaces as their volumes tend to infinity. Our main theorem is a uniform version of the Lück Approximation Theorem \cite{luck}, which is much stronger than the linear upper bounds on Betti numbers given by Gromov in \cite{BGS}. The basic idea is to adapt the theory of local convergence, originally introduced for sequences of graphs of bounded degree by Benjamimi and Schramm, to sequences of Riemannian manifolds. Using rigidity theory we are able to show that when the volume tends to …

Dehn填充的$L^2$-Betti数

$L^2$-Betti numbers of Dehn fillings

本文研究群论Dehn填充的$L^2$-Betti数，证明了对于一大类virtually special群，其充分深的Dehn填充保持$L^2$-Betti数不变，并应用于验证Singer猜想、建立虚拟纤维化准则及构造具有奇异子群的双曲群新例子。

虚拟Euler类为一的准则

A criterion for virtual Euler class one

本文证明了定向闭双曲3-流形中存在满足虚拟Euler类为一的拉紧叶状结构的准则，并构造了支持Yazdi猜想的多个例子。

On the Characterization Problem of Alexander Polynomials of Closed 3-Manifolds

We give a characterization for the Alexander Polynomials of closed orientable 3-manifolds M with first Betti number 1, as well as some partial results for the characterization problem for M having first Betti number > 1. We first prove an analogue of a theorem of Levine: that the product of an Alexander polynomial of M with a symmetric polynomial in the same number of variables having non 0 trace, is again an Alexander polynomial of a closed orientable 3-manifold. Using the fact that there exists M with Alexander polynomial = 1 for M with first Betti number 1, 2 or 3, we conclude that symmetri…

On acylindrical hyperbolicity of groups with positive first $\ell^2$-Betti number

We prove that every finitely presented group with positive first $\ell^2$-Betti number that virtually surjects onto $\mathbb Z$ is acylindrically hyperbolic. In particular, this implies acylindrical hyperbolicity of finitely presented residually finite groups with positive first $\ell^2$-Betti number as well as groups of deficiency at least $2$.

有限体积双曲流形的 Betti 数

On the Betti Numbers of Finite Volume Hyperbolic Manifolds

本文利用酉完整群、尖点上调和形式的单调性不等式及峰值论证，给出紧与有限体积秩一双曲流形 Betti 数和 de Rham 上同调的新上界。

图流形上切触叶状结构的欧拉类

The Euler classes of taut foliations on graph manifolds

本文确定了闭图流形上所有切触叶状结构的欧拉类集合，并证明任意闭无球面图流形的闭对偶瑟斯顿单位球中的整系数类可虚拟实现为欧拉类，同时通过例子说明虚拟条件的必要性及实与整欧拉类为零的差异。

Convergence of normalized Betti numbers in nonpositive curvature

We study the convergence of volume-normalized Betti numbers in Benjamini-Schramm convergent sequences of non-positively curved manifolds with finite volume. In particular, we show that if $X$ is an irreducible symmetric space of noncompact type, $X \neq \mathbb H^3$, and $(M_n)$ is any Benjamini-Schramm convergent sequence of finite volume $X$-manifolds, then the normalized Betti numbers $b_k(M_n)/vol(M_n)$ converge for all $k$. As a corollary, if $X$ has higher rank and $(M_n)$ is any sequence of distinct, finite volume $X$-manifolds, the normalized Betti numbers of $M_n$ converge to the $L^2…

算术局部对称空间的Benjamini-Schramm收敛

Benjamini--Schramm convergence of arithmetic locally symmetric spaces

该研究证明了算术局部对称空间的细部在体积中占比极小，并推导了其贝蒼潮数的渐进行为。

Virtually Symmetric representations and marked Gauss diagrams

In this paper, we define the notion of a virtually symmetric representation of representations of virtual braid groups and prove that many known representations are equivalent to virtually symmetric. Using one such representation, we define the notion of virtual link groups which is an extension of virtual link groups defined by Kauffman. Moreover, we introduce the concept of marked Gauss diagrams as a generalisation of Gauss diagrams and their interpretation in terms of knot-like diagrams. We extend the definition of virtual link groups to marked Gauss diagrams and define their peripheral str…

Symmetric Enhancements of Involutory Virtual Birack Counting Invariants

We consider involutory virtual biracks with good involutions, also known as symmetric involutory virtual biracks. Any good involution on an involutory virtual birack defines an enhancement of the counting invariant. We provide examples demonstrating that the enhancement is stronger than the unenhanced counting invariant.

斜对称增广矩阵与虚拟涂鸦的刻画

Skew-symmetric augmented matrices and a characterization of virtual doodles

本文基于浸入曲线分类与虚拟纽结配边等工作概述涂鸦概念，引入斜对称增广矩阵判定虚拟涂鸦是否非经典，并刻画了经典涂鸦的虚拟化。

对称群虚拟扩张的换位子群与晶体学商群

Commutator subgroups and crystallographic quotients of virtual extensions of symmetric groups

本文研究对称群虚拟扩张的换位子群与晶体学商群，给出纯虚拟三重群及两个换位子群的显式有限表示，并证明 $VL_n/PVL_n^{'}$ 为晶体学群且构造了无穷多个挠元素。

极限群的虚拟同调与直积的有限完备刚性

Virtual homology of limit groups and profinite rigidity of direct products

本文利用虚拟 Betti 数刻画与 pro-$p$ 群秩梯度技术，证明自由群和曲面群直积在有限展示剩余自由群中的有限完备刚性，并推出相关猜想与流形结论。

Representation stability in the level 4 braid group

We investigate the cohomology of the level 4 subgroup of the braid group, namely, the kernel of the mod 4 reduction of the Burau representation at $t=-1$. This group is also equal to the kernel of the mod 2 abelianization of the pure braid group. We give an exact formula for the first Betti number; it is a quartic polynomial in the number of strands. We also show that, like the pure braid group, the first homology satisfies uniform representation stability in the sense of Church and Farb. Unlike the pure braid group, the group of symmetries - the quotient of the braid group by the level 4 subg…

Covering spaces of arithmetic 3-orbifolds

Let G be an arithmetic Kleinian group, and let O be the associated hyperbolic 3-orbifold or 3-manifold. In this paper, we prove that, in many cases, G is large, which means that some finite index subgroup admits a surjective homomorphism onto a non-abelian free group. This has many consequences, including that O has infinite virtual first Betti number and G has super-exponential subgroup growth. Our first result, which forms the basis for the entire paper, is that G is commensurable with a lattice containing the Klein group of order 4. Our second result is that if G has a finite index subgroup…

纯图辫群同调中的表示渐近性

Representation asymptotics in the homology of pure graph braid groups

该研究针对图的有序构型空间同调问题，给出了任意域上渐近Betti数的显式公式，并在特征零情形下得到对称群不可约表示同调渐近重数的显式公式，推进了表示稳定性相关研究。

Topology of 3-manifolds and a class of groups

This paper grew out of an attempt to find a suitable finite sheeted covering of an aspherical 3-manifold so that the cover either has infinite or trivial first homology group. With this motivation we define a new class of groups. These groups are in some sense eventually perfect. We prove results giving several classes of examples of groups which do (not) belong to this class. Also we prove some elementary results on these groups and state two conjectures. A direct application of one of the conjectures to the virtual Betti number conjecture of Thurston is mentioned.

On the growth of L2-invariants of locally symmetric spaces, II: exotic invariant random subgroups in rank one

In the first paper of this series (arxiv.org/abs/1210.2961) we studied the asymptotic behavior of Betti numbers, twisted torsion and other spectral invariants for sequences of lattices in Lie groups G. A key element of our work was the study of invariant random subgroups (IRSs) of G. Any sequence of lattices has a subsequence converging to an IRS, and when G has higher rank, the Nevo-Stuck-Zimmer theorem classifies all IRSs of G. Using the classification, one can deduce asymptotic statments about spectral invariants of lattices. When G has real rank one, the space of IRSs is more complicated. …