Infinite volume and infinite injectivity radius

Group Actions and Dynamics
Event time: 
Monday, February 15, 2021 - 10:15am
Tsachik Gelander
Speaker affiliation: 
Weizmann Institute
Event description: 

Abstract. We answer a question of Margulis by proving the following: Let G be a higher rank simple Lie group and let Λ ≤ G be a discrete subgroup of infinite covolume, then the locally symmetric space Λ\G/K admits injected balls of any radius. This can be considered as a geometric interpretation of the celebrated Margulis normal subgroup theorem. However, it applies to general discrete subgroups not necessarily associated to lattices. Yet, the result is new even for subgroups of infinite index of lattices. We establish similar results for higher rank semisimple groups with Kazhdan’s property (T). We prove a stiffness result for discrete stationary random subgroups in higher rank groups and a stationary variant of the Stuck-Zimmer theorem for higher rank semisimple groups with property (T). We also show that a stationary limit of a measure supported on discrete subgroups is almost surely discrete.

Joint work with Mikolaj Fraczyk