Yongding Zhu Jinhui Xu Yang Yang Naoki Katoh Shin-ichi Tanigawa Geometric Spanner of Objects under L1 Distance. 395-404 2008 COCOON http://dx.doi.org/10.1007/978-3-540-69733-6_39 conf/cocoon/2008 db/conf/cocoon/cocoon2008.html#ZhuXYKT08