Selecting Heavily Covered Points

A collection of geometric selection lemmas is proved, such as the following: For any set P of n points in three-dimensional space and any set ,9 of m spheres, where each sphere passes through a distinct point pair in P, there exists a point x, not necessarily in P, that is enclosed by f2 (m ! (n log )) of the spheres in S. Similar results apply in arbitrary fixed dimensions, and for geometric bodies other than spheres. The results have applications in reducing the size of geometric structures, such as three-dimensional Delaunay triangulations and Gabriel graphs, by adding extra points to their defining sets. Key words, discrete geometry, computational geometry, selecting points, covering, intervals, boxes, spheres, Delaunay triangulations, finite-element meshes, Gabriel graphs AMS subject classifications. 05B99, 51M99, 52A99, 68Q20, 68R05