Order topology

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In mathematics, an order topology is a certain 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

(a, \infty) = \{ x \mid a < x\}
(-\infty, b) = \{x \mid x < b\}

for some a,b in X. This is equivalent to saying that the open intervals

(a,b) = \{x \mid a < x < b\}

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.

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[edit] Induced order topology

If Y is a subset of X, then Y inherits a total order from X. Y therefore has an order topology, the induced order topology. As a subset of X, Y also has a subspace topology. The subspace topology is always finer than the induced order topology, but they are not in general the same.

For example, consider the subset Y = {–1} ∪ {1/n}n∈N in the rationals. Under the subspace topology, the singleton set {–1} is open in Y, but under the induced order topology, any open set containing –1 must contain all but finitely many members of the space.

[edit] An example of a subspace of a linearly ordered space whose topology is not an order topology

Though the subspace topology of Y = {–1} ∪ {1/n}n∈N in the section above is shown to be not generated by the induced order on Y, it is nonetheless an order topology on Y; indeed, in the subspace topology every point is isolated (i.e., singleton {y} is open in Y for every y in Y), so the subspace topology is the discrete topology on Y (the topology in which every subset of Y is an open set), and the discrete topology on any set is an order topology. To define a total order on Y that generates the discrete topology on Y, simply modify the induced order on Y by defining -1 to be the greatest element of Y and otherwise keeping the same order for the other points, so that in this new order (call it say <1) we have 1/n <1 –1 for all n∈N. Then, in the order topology on Y generated by <1, every point of Y is isolated in Y.

We wish to define here a subset Z of a linearly ordered topological space X such that no total order on Z generates the subspace topology on Z, so that the subspace topology will not be an order topology even though it is the subspace topology of a space whose topology is an order topology.

Let Z =\{-1\}\cup\{x\in R: 0 < x < 1\} = \{-1\}\cup (0,1) in the real line. The same argument as before shows that the subspace topology on Z is not equal to the induced order topology on Z, but one can show that the subspace topology on Z cannot be equal to any order topology on Z.

An argument follows. Suppose by way of contradiction that there is some total order < = <1 on Z such that the order topology generated by < is equal to the subspace topology on Z (note that we are not assuming that < is the induced order on Z, but rather an arbitrarily given total order on Z that generates the subspace topology).

Let M=Z\{-1}. Note that, as a topological space, M is just the open unit interval (0,1) (referring to the interval taken with respect to the usual order, but in what follows we write (a,b) to refer to the interval taken with respect to the order relation < = <1). Since {-1} is open in Z, there is some point p in M such that either -1 < p and the interval (-1,p) is empty or p < -1 and the interval (p,-1) is empty. Without loss of generality we can assume that -1 < p and (-1,p) is empty. Then M\{p} = A ∪ B, where A and B are open and disjoint connected subsets of M. By connectedness, no point of Z\B can lie between two points of B, and no point of Z\A can lie between two points of A. Therefore, either A < B or B < A (A < B means a < b for all a in A and b in B). Assume without loss of generality that A < B. Then, since {p} is not open in Z, there is a point a in A such that p < a and (p,a)\subseteq A Then (-1,a)=[p,a)\subseteq {p}∪ A, so that {p}∪A is an open subset of M and hence M = ({p}∪A) ∪ B is the union of two disjoint open subsets of M so M is not connected, a contradiction since, as a topological space, M is just the open unit interval.

A space whose topology is an order topology is called a Linearly Ordered Topological Space (LOTS), and a subspace of a linearly ordered topological space is called a Generalized Ordered Space (GO-space). Thus the example Z above is an example of a GO-space that is not a linearly ordered topological space.

[edit] Left and right order topologies

Several 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.

The left order topology is the standard topology used for many set-theoretic purposes on a Boolean algebra.

[edit] Ordinal space

For any ordinal number λ one can consider the spaces of ordinal numbers

[0,\lambda) = \{\alpha \mid \alpha < \lambda\}\,
[0,\lambda] = \{\alpha \mid \alpha \le \lambda\}\,

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; otherwise (for finite ordinals), the order topology is simply the discrete topology.

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

[edit] Topology and ordinals

[edit] Ordinals as topological spaces

Any ordinal number can be made into a topological space by endowing it with the order topology (since, being well-ordered, an ordinal is in particular totally ordered): in the absence of indication to the contrary, it is always that order topology which is meant when an ordinal is thought of as a topological space. (Note that if we are willing to accept a proper class as a topological space, then the class of all ordinals is also a topological space for the order topology.)

The set of limit points of an ordinal α is precisely the set of limit ordinals less than α. Successor ordinals (and zero) less than α are isolated points in α. In particular, the finite ordinals and ω are discrete topological spaces, and no ordinal beyond that is discrete. The ordinal α is compact as a topological space if and only if α is a successor ordinal.

The closed sets of a limit ordinal α are just the closed sets in the sense that we have already defined, namely, those which contain a limit ordinal whenever they contain all sufficiently large ordinals below it.

Any ordinal is, of course, an open subset of any further ordinal. We can also define the topology on the ordinals in the following inductive way: 0 is the empty topological space, α+1 is obtained by taking the one-point compactification of α (if α is a limit ordinal; if it is not, α+1 is merely the disjoint union of α and a point), and for δ a limit ordinal, δ is equipped with the inductive limit topology.

As topological spaces, all the ordinals are Hausdorff and even normal. They are also totally disconnected (connected components are points), scattered (=every non-empty set has an isolated point; in this case, just take the smallest element), zero-dimensional (=the topology has a clopen basis: here, write an open interval (β,γ) as the union of the clopen intervals (β,γ'+1)=[β+1,γ'] for γ'<γ). However, they are not extremally disconnected in general (there is an open set, namely ω, whose closure is not open).

The topological spaces ω1 and its successor ω1+1 are frequently used as text-book examples of non-countable topological spaces. For example, in the topological space ω1+1, the element ω1 is in the closure of the subset ω1 even though no sequence of elements in ω1 has the element ω1 as its limit. The space ω1 is first-countable, but not second-countable, and ω1+1 has neither of these two properties, despite being compact. It is also worthy of note that any continuous function from ω1 to R (the real line) is eventually constant: so the Stone-Čech compactification of ω1 is ω1+1, just as its one-point compactification (in sharp contrast to ω, whose Stone-Čech compactification is much larger than ω1).

[edit] Ordinal-indexed sequences

If α is a limit ordinal and X is a set, an α-indexed sequence of elements of X merely means a function from α to X. If X is a topological space, we say that an α-indexed sequence of elements of X converges to a limit x when it converges as a net, in other words, when given any neighborhood U of x there is an ordinal β<α such that xι is in U for all ι≥β. This coincides with the notion of limit defined above for increasing ordinal-indexed sequences in an ordinal.

Ordinal-indexed sequences are more powerful than ordinary (ω-indexed) sequences to determine limits in topology: for example, ω1 is a limit point of ω1+1 (because it is a limit ordinal), and, indeed, it is the limit of the ω1-indexed sequence which maps any ordinal less than ω1 to itself: however, it is not the limit of any ordinary (ω-indexed) sequence in ω1, since any function from the natural numbers to ω1 is bounded. However, ordinal-indexed sequences are not powerful enough to replace nets (or filters) in general: for example, on the Tychonoff plank (the product space (\omega_1+1)\times(\omega+1)), the corner point (ω1,ω) is a limit point (it is in the closure) of the open subset \omega_1\times\omega, but it is not the limit of an ordinal-indexed sequence.

[edit] See also

[edit] References

  • Stephen Willard, General Topology, (1970) Addison-Wesley Publishing Company, Reading Massachusetts.
  • This article incorporates material from Order topology on PlanetMath, which is licensed under the GFDL.
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