Abstract
We prove Zimmer’s conjecture for \(C^2\) actions by finite-index subgroups of \(\mathrm {SL}(m,{\mathbb {Z}})\) provided \(m>3\). The method utilizes many ingredients from our earlier proof of the conjecture for actions by cocompact lattices in \(\mathrm {SL}(m,{\mathbb {R}})\) (Brown et al. in Zimmer’s conjecture: subexponential growth, measure rigidity, and strong property (T), 2016. arXiv:1608.04995) but new ideas are needed to overcome the lack of compactness of the space \((G \times M)/\Gamma \) (admitting the induced G-action). Non-compactness allows both measures and Lyapunov exponents to escape to infinity under averaging and a number of algebraic, geometric, and dynamical tools are used control this escape. New ideas are provided by the work of Lubotzky, Mozes, and Raghunathan on the structure of nonuniform lattices and, in particular, of \(\mathrm {SL}(m,{\mathbb {Z}})\) providing a geometric decomposition of the cusp into rank one directions, whose geometry is more easily controlled. The proof also makes use of a precise quantitative form of non-divergence of unipotent orbits by Kleinbock and Margulis, and an extension by de la Salle of strong property (T) to representations of nonuniform lattices.
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Notes
After this work was completed, Brown-Damjanovic-Zhang showed that some modifications of our arguments also give a proof for \(C^1\) diffeomorphisms [3].
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Acknowledgements
We thank Dave Witte Morris for his generous willingness to answer questions of all sorts throughout the production of this paper and [4]. We also thank to Shirali Kadyrov, Jayadev Athreya and Alex Eskin for helpful conversations, particularly on the material in Sect. 1.5 and Mikael de la Salle for many helpful conversations regarding strong property (T). We also thank the anonymous referee for a very careful reading and numerous comments which helped improve the exposition.
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David Fisher was partially supported by NSF Grants DMS-1308291 and DMS-1607041. David Fisher was also partially supported by the University of Chicago, and by NSF Grants DMS 1107452, 1107263, 1107367, “RNMS: Geometric Structures and Representation Varieties” (the GEAR Network) during a visit to the Isaac Newton Institute in Cambridge.
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Brown, A., Fisher, D. & Hurtado, S. Zimmer’s conjecture for actions of \(\mathrm {SL}(m,\pmb {\mathbb {Z}})\). Invent. math. 221, 1001–1060 (2020). https://doi.org/10.1007/s00222-020-00962-x
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DOI: https://doi.org/10.1007/s00222-020-00962-x