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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,$\Pi)$, 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,$\Pi)$. We also introduce the notions of upper and lower approximation of a set in the space (U,$\Pi)$, 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 {\it 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.

03E20Other classical set theory (logic)