... −111;0sj.4.12 Lemma.Γkis of finite index in SL(2, ) (12)2k.Proof. The subgroup Γθ2k⊂ Γkis of finite index in SL(2, )2kandthus also in SL(2,) (12)2k.434 JENS MARKLOF4.13. Remark. Note ... Theorem 1.4 in [27] on the equidistribution of translates of unipotent orbits. Because of thesimple structure of the Lie groups studied here, the proof of Theorem 5.7 isless involved than in the ... to SL(2, )2k.4.14. In this paper, we will be interested in the case of quadratic forms in twovariables, i.e., k =2. The corresponding theta sum (defined for general k in 4.5) reads thenΘf(τ,φ;...