Marek Cygan Fabrizio Grandoni Stefano Leonardi Marcin Mucha Marcin Pilipczuk Piotr Sankowski Approximation Algorithms for Union and Intersection Covering Problems. 28-40 2011 FSTTCS http://dx.doi.org/10.4230/LIPIcs.FSTTCS.2011.28 conf/fsttcs/2011 db/conf/fsttcs/fsttcs2011.html#CyganGLMPS11