Asymptotic behaviour of trajectories of unipotent flows on homogeneous spaces

Abstract

We show that ifG is a semisimple algebraic group defined overQ and Gamma is an arithmetic lattice inG:=G R with respect to theQ-structure, then there exists a compact subsetC ofG/Gamma such that, for any unipotent one-parameter subgroup {u t} ofG and anygε G, the time spent inC by the {u t}-trajectory ofgGamma, during the time interval [0,T], is asymptotic toT, unless {g ^-1utg} is contained in aQ-parabolic subgroup ofG. Some quantitative versions of this are also proved. The results strengthen similar assertions forSL(n,Z),n≥ 2, proved earlier in [5] and also enable verification of a technical condition introduced in [7] for lattices inSL(3,R), which was used in our proof of Raghunathan's conjecture for a class of unipotent flows, in [8].