Pankaj K. Agarwal Esther Ezra Micha Sharir Near-linear approximation algorithms for geometric hitting sets. 23-32 2009 Symposium on Computational Geometry http://doi.acm.org/10.1145/1542362.1542368 conf/compgeom/2009 db/conf/compgeom/compgeom2009.html#AgarwalES09