Glossary of Topology - A

A

Absolutely closed

See H-closed

Accessible

See .

Accumulation point

See limit point.

Alexandrov topology

A space X has the Alexandrov topology (or is finitely generated) if arbitrary intersections of open sets in X are open, or equivalently, if arbitrary unions of closed sets are closed, or, again equvalently, if the open sets are the upper sets of a poset.

Almost discrete

A space is almost discrete if every open set is closed (hence clopen). The almost discrete spaces are precisely the finitely generated zero-dimensional spaces.

Approach space

An approach space is a generalization of metric space based on point-to-set distances, instead of point-to-point.