Various expanded rough set models based on tolerance relations enlarge the application fields of rough set theory. Through generating tolerance relations to fuzzy tolerance relations and combining with dominance relations, a tolerance class of a fuzzy tolerance relation is further decomposed into a positive fuzzy tolerance class, a negative fuzzy tolerance class and a purely fuzzy tolerance class. Then the paper gives out relative definitions of lower approximation and upper approximation respectively. Furthermore it proposes the decomposition of rough sets of fuzzy tolerance relations. This method can cope with continuous attributes effectively. Information retrieval, as one of its example applications, is introduced to show its meaningfulness.

Information retrieval,rough set, fuzzy set, fuzzy tolerance relation, dominance relation