Ronald L. Graham Jeffrey C. Lagarias Colin L. Mallows Allan R. Wilks Catherine H. Yan Apollonian Circle Packings: Geometry and Group Theory III. Higher Dimensions. 37-72 2006 35 Discrete & Computational Geometry 1 http://dx.doi.org/10.1007/s00454-005-1197-8 db/journals/dcg/dcg35.html#GrahamLMWY06a