Order topology
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In mathematics, the order topology is a topology that can be defined on any totally ordered set. It is a natural generalization of the topology of the real numbers to arbitrary totally ordered sets.
If X is a totally ordered set, the order topology on X is generated by the subbase of open rays
- <math>(a, \infty) = \{ x \mid a < x\}<math>
- <math>(-\infty, b) = \{x \mid x < b\}<math>
for some a,b in X. This is equivalent to saying that the open intervals
- <math>(a,b) = \{x \mid a < x < b\}<math>
together with the above rays form a basis for the order topology. The open sets in X are the sets that are a union of (possibly infinitely many) such open intervals and rays.
The order topology makes X into a completely normal Hausdorff space.
The standard topologies on R, Q, and N are the order topologies.
Ordinal space
For any ordinal number λ one can consider the spaces of ordinal numbers
- <math>[0,\lambda) = \{\alpha \mid \alpha < \lambda\}\,<math>
- <math>[0,\lambda] = \{\alpha \mid \alpha \le \lambda\}\,<math>
together with the natural order topology. These spaces are called ordinal spaces. (Note that in the usual set-theoretic construction of ordinal numbers we have λ = [0,λ) and λ + 1 = [0,λ]). Obviously, these spaces are mostly of interest when λ is an infinite ordinal.
When λ = ω (the first infinite ordinal), the space [0,ω) is just N with the usual topology, while [0,ω] is the one-point compactification of N.
Of particular interest is the case when λ = ω1, the first uncountable ordinal. The element ω1 is a limit point of the subset [0,ω1) even though no sequence of elements in [0,ω1) has the element ω1 as its limit. In particular, [0,ω1] is not first-countable. The subspace [0,ω1) is first-countable however, since the only point without a countable local base is ω1. Some further properties include
- neither [0,ω1) or [0,ω1] is separable or second-countable
- [0,ω1] is compact while [0,ω1) is sequentially compact and countably compact, but not compact or paracompact
Left and right order topologies
Several interesting variants of the order topology can be given:
- The left order topology on X is the topology whose open sets consist of intervals of the form (a, ∞).
- The right order topology on X is the topology whose open sets consist of intervals of the form (−∞, b).
The left and right order topologies can be used to give counterexamples in general topology. For example, the left or right order topology on a bounded set provides an example of a compact space that is not Hausdorff.