A subset \(U\) of a metric (or topological) space^{1} \(X\) is called **compact**, if for every open cover $(U_i)_{i\in I}$ of $U$ there exist only finitely many indices \(i_1,i_2,\ldots,i_k\in I\) with

\[U\subset U_{i_1}\cup U_{i_2}\cup \ldots \cup U_{i_n}.\]

- The finite union $U_{i_1}\cup U_{i_2}\cup \ldots \cup U_{i_n}$ is sometimes referred to as an
**open subcover**. - The existence of a finite subcover for every open cover of $U$ is called the
**Heine-Borel property**of \(U\) (and is due to Heinrich Eduard Heine and Émile Borel.

^{1} The above definition makes no reference to any kind of a metric of the space $X$. Thus, it can be used in more generalized topological spaces rather than metric spaces.

| | | | | created: 2017-02-20 23:43:11 | modified: 2017-03-12 14:01:23 | by: *bookofproofs* | references: [582]

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[582] **Forster Otto**: “Analysis 2, Differentialrechnung im \(\mathbb R^n\), Gewöhnliche Differentialgleichungen”, Vieweg Studium, 1984