Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In mathematics, a cover of a set X is a collection of sets such that X is a subset of the union of sets in the collection. Covers are commonly used in the context of topology. If the set X is a topological space, then a cover C of X is a collection of subsets U of X whose union is the whole space X. In this case we say that C covers X, or that the sets U cover X.

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