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A set is open if every point in is an interior point. A set is closed if it contains all of its b...

More generally, one defines open sets as the members of a given collection of subsets of a given ...

. In one-space, the open set is an open interval. In two-space, the open set is a disk. In three-...

一個集合中隨便指定那些subsets 是open sets 是沒有意義的. 由Proposition 1.1.5 和. Proposition 1.1.6, 我們知道在實數上的open set ...