Sorgenfrey plane

In topology, the Sorgenfrey plane is a frequently-cited counterexample to many otherwise plausible-sounding conjectures. It consists of the product of two copies of the Sorgenfrey line, which is the real line R under the half-open interval topology. The Sorgenfrey line and plane are named for the American mathematician Robert Sorgenfrey.

A basis for the Sorgenfrey plane, denoted X from now on, is therefore the set of rectangles that include the west edge, southwest corner, and south edge, and omit the southeast corner, east edge, northeast corner, north edge, and northwest corner. Open sets in X are unions of such rectangles.

X is an example of a space that is a product of Lindelöf spaces that is not itself a Lindelöf space. It is also an example of a space that is a product of normal spaces that is not itself normal. The so-called anti-diagonal ={ ("x", −"x") | "x" ∈ R } is a
discrete subset of this space, and this is a non-separable subsetof the separable space X. It shows that separability does not inheritto subspaces. Note that K={ ("x", −"x") | "x" ∈ Q } andK are closed sets that cannot be separated by open sets, showing that X is not normal.

References

* John L. Kelley, "General topology", van Nostrand, 1955. Pp.133-134.
* Robert Sorgenfrey, "On the topological product of paracompact spaces", "Bull. Amer. Math. Soc." 53 (1947) 631-632.
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