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【多选题】
根据点集所属点的特征,可进一步定义一些重要的平面点集,具体包括( ):A.
若点集E内 任意一点均为E的 内点，则称E为 开集
B.
若点集E的 开集，则称E为
C.
若点集E内 的任何两点，都可用折线连结起来，且该折线上的点都属于E， 则称E为 连通集
D.
连通的开集称为 区域 或 开区域 。
E.
开区域连同它的边界一起称为 闭区域
F.
一个点集若不是有界集，就称它为 无界集。
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