Julien Demouth Olivier Devillers Marc Glisse Xavier Goaoc Helly-type theorems for approximate covering. 120-128 2008 Symposium on Computational Geometry http://doi.acm.org/10.1145/1377676.1377696 conf/compgeom/2008 db/conf/compgeom/compgeom2008.html#DemouthDGG08