HEY.

For a subset S of Euclidean space Rⁿ, the following two statements are equivalent:

In the context of real analysis the former property is sometimes used as the defining property of compactness. However, the two definitions cease to be equivalent when we consider subsets of more general metric spaces and in this generality only the latter property is used to define compactness. In fact, the Heine?Borel theorem for arbitrary metric spaces reads: