Perpendicular Dissections of Space

For each pair (Qi, Qj) of reference points and each real number r there is a unique hyperplane h ⊥ QiQj such that d(P,Qi)2 − d(P,Qj)2 = r for points P in h. Take n reference points in d-space and for each pair (Qi, Qj) a finite set of real numbers. The corresponding perpendiculars form an arrangement of hyperplanes. We explore the structure of the semilattice of intersections of the hyperplanes for generic reference points. The main theorem is that there is a real, additive gain graph (this is a graph with an additive real number associated invertibly to each edge) whose set of balanced flats has the same structure as the intersection semilattice. We examine the requirements for genericity, which are related to behavior at infinity but remain mysterious; also, variations in the construction rules for perpendiculars. We investigate several particular arrangements with a view to finding the exact numbers of faces of each dimension. The prototype, the arrangement of all perpendicular bisectors, was studied by Good and Tideman, motivated by a geometric voting theory. Most of our particular examples are suggested by extensions of that theory in which voters exercise finer discrimination. Throughout, we propose many research problems. Postpublication revision 8 May 2002: added reference to Voronoi in §9: “that goes back to the original paper [26]”.