Prosenjit Bose Paz Carmi Mathieu Couture Anil Maheshwari Pat Morin Michiel H. M. Smid Spanners of Complete k-Partite Geometric Graphs. 1803-1820 2009 38 SIAM J. Comput. 5 http://dx.doi.org/10.1137/070707130 db/journals/siamcomp/siamcomp38.html#BoseCCMMS09