Moses Charikar MohammadTaghi Hajiaghayi Howard J. Karloff Improved Approximation Algorithms for Label Cover Problems. 23-34 2009 ESA http://dx.doi.org/10.1007/978-3-642-04128-0_3 conf/esa/2009 db/conf/esa/esa2009.html#CharikarHK09