Quasi-actions on trees I. Bounded valence

Quasi-Isometric Bounded Generation by ${\mathbb Q}$-Rank-One Subgroups

We say that a subset $X$ quasi-isometrically boundedly generates a finitely generated group $Γ$ if each element $γ$ of a finite-index subgroup of $Γ$ can be written as a product $γ= x_1 x_2 \cdots x_r$ of a bounded number of elements of $X$, such that the word length of each $x_i$ is bounded by a constant times the word length of $γ$. A. Lubotzky, S. Mozes, and M.S. Raghunathan observed in 1993 that ${\rm SL}(n,{\mathbb Z})$ is quasi-isometrically boundedly generated by the elements of its natural ${\rm SL}(2,{\mathbb Z})$ subgroups. We generalize (a slightly weakened version of) this by showi…

Infinite sums of Brooks quasimorphisms and cup products in bounded cohomology

We study a class of quasimorphisms of the free group that can be expressed as infinite sums of Brooks quasimorphisms with some nice properties. We then review Heuer's framework of decompositions developed in arXiv:1710.03193, and put these quasimorphisms in that context. This allows us to prove triviality of a number of cup products in the bounded cohomology of the free group with trivial real coefficients. Finally, we introduce free products of decompositions and relate them to Rolli's quasimorphisms of free products. A technical result concerning these, which remains an open question, implie…

Trivial cup products in bounded cohomology of the free group via aligned chains

We prove that the cup product of $Δ$-decomposable quasimorphisms, Brooks quasimorphisms or Rolli quasimorphisms with any bounded cohomology class of arbitrary positive degree is trivial.

拟轨道与树拟等距的拟作用

Quasi-actions whose quasi-orbits are quasi-isometric to trees

本文给出了任意群在度量空间上的拟作用可归约为有界度数树上有界等距作用的充要条件，并证明若拟作用度量正则且拟轨道与树拟等距，则该群是虚拟自由的。

Maximally symmetric trees

We characterize the ``best'' model geometries for the class of virtually free groups, and we show that there is a countable infinity of distinct ``best'' model geometries in an appropriate sense--these are the maximally symmetric trees. The first theorem gives several equivalent conditions on a bounded valence, cocompact tree T without valence 1 vertices saying that T is maximally symmetric. The second theorem gives general constructions for maximally symmetric trees, showing for instance that every virtually free group has a maximally symmetric tree for a model geometry.

Bijective Quasi-Isometries of Amenable Groups

Whyte showed that any quasi-isometry between non-amenable groups is a bounded distance from a bijection. In contrast this paper shows that for amenable groups, inclusion of a proper subgroup of finite index is never a bounded distance from a bijection.