@inproceedings{dennis1981disjoint,
title={Disjoint spheres, approximation by imaginary quadratic numbers, and the logarithm law for geodesics},
author={Dennis Sullivan},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203258321502326},
booktitle={Acta Mathematica},
volume={149},
number={1},
pages={215-237},
year={1981},
}
Dennis Sullivan. Disjoint spheres, approximation by imaginary quadratic numbers, and the logarithm law for geodesics. 1981. Vol. 149. In Acta Mathematica. pp.215-237. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203258321502326.