Boris Bukh Jirí Matousek Gabriel Nivasch Lower bounds for weak epsilon-nets and stair-convexity. 1-10 2009 Symposium on Computational Geometry http://doi.acm.org/10.1145/1542362.1542365 conf/compgeom/2009 db/conf/compgeom/compgeom2009.html#BukhMN09