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A topological space $F$ consisting of two points. If both one-point subsets in $F$ are open (both are then closed), $F$ is said to be a simple colon. If only one one-point subset in $F$ is open, $F$ is said to be a connected colon. Finally, if only the empty subset and all of $F$ in $F$ are open, $F$ is called an identified colon; this space — unlike the first two, which are very important though simple — has found no applications.


[1] P.S. Aleksandrov, "Einführung in die Mengenlehre und die allgemeine Topologie" , Deutsch. Verlag Wissenschaft. (1984) (Translated from Russian)


The term two-point discrete space is often applied to the simple colon. Its topological powers are called Cantor cubes. These spaces are universal in two ways: Every zero-dimensional compactum can be imbedded into a Cantor cube of the same weight, and every compactum can be obtained as the continuous image of a closed set of a Cantor cube of the same weight. These facts generalize well-known results on the Cantor cube of countable weight, the Cantor set. The connected colon is also known as Sierpinski space. Its topological powers are called Alexandrov cubes; they are universal in that they contain all $T_0$-spaces topologically.


[a1] R. Engelking, "General topology" , PWN (1977) (Translated from Polish)
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Colon. Encyclopedia of Mathematics. URL:
This article was adapted from an original article by A.A. Mal'tsev (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article