#### Volume 23, issue 4 (2019)

 Recent Issues
 The Journal About the Journal Editorial Board Editorial Interests Subscriptions Submission Guidelines Submission Page Policies for Authors Ethics Statement ISSN (electronic): 1364-0380 ISSN (print): 1465-3060 Author Index To Appear Other MSP Journals
Hyperbolicity as an obstruction to smoothability for one-dimensional actions

### Christian Bonatti, Yash Lodha and Michele Triestino

Geometry & Topology 23 (2019) 1841–1876
##### Abstract

Ghys and Sergiescu proved in the 1980s that Thompson’s group $T\phantom{\rule{0.3em}{0ex}}$, and hence $F\phantom{\rule{0.3em}{0ex}}$, admits actions by ${C}^{\infty }$ diffeomorphisms of the circle. They proved that the standard actions of these groups are topologically conjugate to a group of ${C}^{\infty }$ diffeomorphisms. Monod defined a family of groups of piecewise projective homeomorphisms, and Lodha and Moore defined finitely presentable groups of piecewise projective homeomorphisms. These groups are of particular interest because they are nonamenable and contain no free subgroup. In contrast to the result of Ghys and Sergiescu, we prove that the groups of Monod and Lodha and Moore are not topologically conjugate to a group of ${C}^{1}$ diffeomorphisms.

Furthermore, we show that the group of Lodha and Moore has no nonabelian ${C}^{1}$ action on the interval. We also show that many of Monod’s groups $H\left(A\right)$, for instance when $A$ is such that $\mathsf{PSL}\left(2,A\right)$ contains a rational homothety $x↦\frac{p}{q}x$, do not admit a ${C}^{1}$ action on the interval. The obstruction comes from the existence of hyperbolic fixed points for ${C}^{1}$ actions. With slightly different techniques, we also show that some groups of piecewise affine homeomorphisms of the interval or the circle are not smoothable.

##### Keywords
group actions on the interval, piecewise-projective homeomorphisms, hyperbolic dynamics
##### Mathematical Subject Classification 2010
Primary: 37C85, 57M60
Secondary: 37D40, 37E05, 43A07