“…Heilbronn asked for the exact value or approximation of ∆ n . The list [1,2,3,4,7,8,13,14,17,18,19,20,21,22,23,24,25,26,27] is a selection of papers dealing with the problem. Obviously, the value of ∆ n will change only by a small constant factor for every unit area convex shape, and it has become customary to consider the unit square [22].…”