books.google.com.au - Lattices are discrete subgroups of maximal rank in a Euclidean space. To each such geometrical object, we can attach a canonical sphere packing which, assuming some regularity, has a density. The question of estimating the highest possible density of a sphere packing in a given dimension is a fascinating...http://books.google.com.au/books/about/Perfect_Lattices_in_Euclidean_Spaces.html?id=gd9CcFclBRIC&utm_source=gb-gplus-sharePerfect Lattices in Euclidean Spaces
