Tsau Young Lin Qing Liu Y. Y. Yao Logics Systems for Approximate Reasoning: Approximation via Rough Sets and Topological Spaces. 65-74 1994 conf/ismis/1994 ISMIS db/conf/ismis/ismis94.html#LinLY94 http://dx.doi.org/10.1007/3-540-58495-1_7