Unsmoothable group actions on compact one-manifolds

Abstract

We show that no finite index subgroup of a sufficiently complicated mapping class group or braid group can act faithfully by $C^{1+\mathrm{bv}}$ diffeomorphisms on the circle, which generalizes a result of Farb-Franks, and which parallels a result of Ghys and Burger-Monod concerning differentiable actions of higher rank lattices on the circle. This answers a question of Farb, which has its roots in the work of Nielsen. We prove this result by showing that if a right-angled Artin group acts faithfully by $C^{1+\mathrm{bv}}$ diffeomorphisms on a compact one-manifold, then its defining graph has no subpath of length three. As a corollary, we also show that no finite index subgroup of $\text{Aut}(F_n)$ and $\text{Out}(F_n)$ for $n\ge 3$, the Torelli group for genus at least $3$, and of each term of the Johnson filtration for genus at least $5$, can act faithfully by $C^{1+\mathrm{bv}}$ diffeomorphisms on a compact one-manifold.

22 pages, incorporated referee's comments. To appear in J. Eur. Math. Soc