Peng Zhang Jin-yi Cai Linqing Tang Wenbo Zhao Approximation and Hardness Results for Label Cut and Related Problems. 460-469 2009 TAMC http://dx.doi.org/10.1007/978-3-642-02017-9_48 conf/tamc/2009 db/conf/tamc/tamc2009.html#ZhangCTZ09