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Approximations in the space (U,${\Pi }$ ). (English) Zbl 0553.04002
In this paper we introduce the notion of an approximation space as a pair (U,${\Pi }\right)$, where U denotes an arbitrary, non-empty set and ${\Pi }$ denotes a covering of U. The elements of the covering ${\Pi }$ we call elementary sets in the approximation space (U,${\Pi }\right)$. We also introduce the notions of upper and lower approximation of a set in the space (U,${\Pi }\right)$, rough equality, rough inclusion, rough relation, and the notion of the approximation of a function. A theory of approximation which is based on these notions is a generalization of the theory of approximation in the sense of the papers by Z. Pawlak [Int. J. Comput. Inf. Sci. 11, 341-356 (1982; Zbl 0501.68053), Pr. Inst. Podstaw Inf. Pol. Akad. Nauk 435 (1981) and ibid. 467 (1981; Zbl 0516.04001)] and the author [Demonstr. Math. 15, 1129-1133 (1982; Zbl 0526.04005)]. In a special case when the covering ${\Pi }$ is a partition, these two theories are identical.
##### MSC:
 3e+20 Other classical set theory (logic)
##### Keywords:
rough sets; approximation space; rough relation