Abstract
Generalized approximation algebras and approximation algebras are defined as a theoretical counterpart of all the situations where a "lower" and an "upper" mapping are used. Some models of these structures are discusse, among them rough sets, fuzzy rough sets and possibility theory. Generalized approximation framework and approximation framework are also introduced as an abstraction of all those cases where several approximations are possibile on the same element. Also in this case some examples are given.
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