Abstract. During the last decades, by applying techniques from Number Theory, Combinatorics and Measure Theory, remarkable progress has been made in the study of deep holes, free planes and related topics in packings of convex bodies, especially in lattice ball packings. Meanwhile, some fascinating new problems have been proposed. To stimulate further research in related areas, we will review the main results, some key techniques and some fundamental problems about deep holes, free cylinders and free planes in this paper. Let E n denote the n-dimensional Euclidean space and let x 1 , x 2 denote the Euclidean distance between two points x 1 and x 2 . For convenience, we call the minimum distance between distinct points of a set its separation. Based on Question 0.1 it is natural to ask the following question. By a routine argument one can show that the minimum is 2/ √ 3 and every optimal set is isometric to
Background and exampleswhere Z indicates the integer ring. In this case, one can triangulate E 2 into congruent regular triangles of edge length 2 and with all their vertices belonging to Λ 2 .