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of members of belongs to
# The intersection of any finite number of members of belongs to

As this definition of a topology is the most commonly used, the set of the open sets is commonly called a topology on

A subset is said to be closed in if its complement is an open set. Note that from this definition, it follows that the empty set and are simultaneously open and closed - that is, the two sets are complements of one another, while each of them is, itself,

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