H-closed space

In mathematics, a Hausdorff space is said to be H-closed, or Hausdorff closed, or absolutely closed if it is closed in every Hausdorff space containing it as a subspace. This property is a generalization of compactness, since a compact subset of a Hausdorff space is closed. Thus, every compact Hausdorff space is H-closed. The notion of an H-closed space has been introduced in 1924 by P. Alexandroff and P. Urysohn.

Examples and equivalent formulations

• The unit interval ${\displaystyle [0,1]}$, endowed with the smallest topology which refines the euclidean topology, and contains ${\displaystyle Q\cap [0,1]}$ as an open set is H-closed but not compact.
• Every regular Hausdorff H-closed space is compact.
• A Hausdorff space is H-closed if and only if every open cover has a finite subfamily with dense union.

See also

• Compact space