Pankaj K. Agarwal Herbert Edelsbrunner Otfried Schwarzkopf Euclidean Minimum Spanning Trees and Bichromatic Closest Pairs. 407-422 1991 6 Discrete & Computational Geometry db/journals/dcg/dcg6.html#AgarwalES91 http://dx.doi.org/10.1007/BF02574698