Inmathematics, theArens–Fort space is a special example in the theory oftopological spaces, named forRichard Friederich Arens andM. K. Fort, Jr.

The Arens–Fort space is the topological space${\displaystyle (X,\tau )}$ where${\displaystyle X}$ is the set of ordered pairs of non-negativeintegers${\displaystyle (m,n).}$ A subset${\displaystyle U\subseteq X}$ isopen, that is, belongs to${\displaystyle \tau ,}$ if and only if:

• ${\displaystyle U}$ does not contain${\displaystyle (0,0),}$ or
• ${\displaystyle U}$ contains${\displaystyle (0,0)}$ and also all but a finite number of points of all but a finite number of columns, where a column is a set${\displaystyle \{(m,n):0\le n\in \mathbb{Z}\}}$ with${\displaystyle 0\le m\in \mathbb{Z}}$ fixed.

In other words, an open set is only "allowed" to contain${\displaystyle (0,0)}$ if only a finite number of its columns contain significant gaps, where a gap in a column is significant if it omits an infinite number of points.

It is

• Hausdorff
• regular
• normal

It is not:

• second-countable
• first-countable
• metrizable
• compact
• sequential
• Fréchet–Urysohn

There is no sequence in${\displaystyle X\setminus \{(0,0)\}}$ that converges to${\displaystyle (0,0).}$ However, there is a sequence${\displaystyle x_{\bullet }={\left(x_i\right)}_{i=1}^{\mathrm{\infty }}}$ in${\displaystyle X\setminus \{(0,0)\}}$ such that${\displaystyle (0,0)}$ is acluster point of${\displaystyle x_{\bullet }.}$

• Fort space – Examples of topological spaces
• List of topologies – List of concrete topologies and topological spaces

• Steen, Lynn Arthur;Seebach, J. Arthur Jr. (1995) [1978],Counterexamples in Topology (Dover reprint of 1978 ed.), Berlin, New York:Springer-Verlag,ISBN 978-0-486-68735-3,MR 0507446

• Topological spaces

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• Short description matches Wikidata