Suhua Zheng Xiaodong Liu Fenglan Xiong The Relativity and Properties of Logical Operator, Fuzzy Relationsand General Fuzzy Rough Approximation. 475-478 2008 FSKD (1) http://dx.doi.org/10.1109/FSKD.2008.388 conf/fskd/2008-1 db/conf/fskd/fskd2008-1.html#ZhengLX08