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Prove: a set is closed if and only if it contains all its limit points given that
(1) a point is said to be a limit point of (Omega) and if there exists a sequence of points (Z_ninOmega) such that (Z_n e Z) and (lim_{x o infty} Z_n=Z)
(2) a set (Omega) is closed if its complement (Omega^c = C-Omega)
(3) a set (Omega) is open if every point in that set is an interior point of (Omega)
(4) a point is an interior point of (Omega subset C) if there exists (r > 0) such that (D_r(Z_0) subset Omega)

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原文地址:https://www.cnblogs.com/treyscience/p/4536932.html