Type-2 fuzzy rough sets based on extended t-norms

This paper is devoted to investigate type-2 fuzzy rough sets based on extended t-norms with respect to type-2 fuzzy relations with convex normal fuzzy truth values. Type-2 fuzzy rough sets are proposed on two finite universes of discourse and characterized from both constructive and axiomatic approaches. In the constructive approach, different classes of type-2 fuzzy rough sets are investigated. An approximate algorithm of type-2 fuzzy rough sets based on a-cut sets is given with numerical examples. In the axiomatic approach, the axiomatic characterizations of type-2 fuzzy rough approximation operators are studied. Moreover, the topological properties of type-2 fuzzy rough sets are discussed. In the end, this paper introduces interval-valued type-2 fuzzy rough sets and discusses their properties. (C) 2015 Elsevier Inc. All rights reserved.