# Order topology

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"

for all *a, b* in *X*. Provided *X* has at least two elements, this is equivalent to saying that the open intervals

together with the above rays form a base 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.

A topological space *X* is called **orderable** if there exists a total order on its elements such that the order topology induced by that order and the given topology on *X* coincide. The order topology makes *X* into a completely normal Hausdorff space.

If *Y* is a subset of *X*, *X* a totally ordered set, then *Y* inherits a total order from *X*. The set *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 at least as fine as the induced order topology, but they are not in general the same.

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.

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.^{[clarification needed]}

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

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 (still discrete) topology, while [0,ω] is the one-point compactification of **N**.

Of particular interest is the case when *λ* = ω_{1}, the set of all countable ordinals, and the first uncountable ordinal. The element ω_{1} is a limit point of the subset [0,ω_{1}) even though no sequence of elements in [0,ω_{1}) has the element ω_{1} as its limit. In particular, [0,ω_{1}] is not first-countable. The subspace [0,ω_{1}) is first-countable however, since the only point in [0,ω_{1}] without a countable local base is ω_{1}. Some further properties include

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 that 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 that 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*, and for *δ* a limit ordinal, *δ* is equipped with the inductive limit topology. Note that if *α* is a successor ordinal, then *α* is compact, in which case its one-point compactification *α*+1 is the disjoint union of *α* and a point.

As topological spaces, all the ordinals are Hausdorff and even normal. They are also totally disconnected (connected components are points), scattered (every non-empty subspace 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 are open sets, for example the even numbers from ω, whose closure is not open).

The topological spaces ω_{1} and its successor ω_{1}+1 are frequently used as text-book examples of non-countable topological spaces.
For example, in the topological space ω_{1}+1, the element ω_{1} is in the closure of the subset ω_{1} even though no sequence of elements in ω_{1} has the element ω_{1} as its limit: an element in ω_{1} is a countable set; for any sequence of such sets, the union of these sets is the union of countably many countable sets, so still countable; this union is an upper bound of the elements of the sequence, and therefore of the limit of the sequence, if it has one.

The space ω_{1} is first-countable, but not second-countable, and ω_{1}+1 has neither of these two properties, despite being compact. It is also worthy of note that any continuous function from ω_{1} to **R** (the real line) is eventually constant: so the Stone–Čech compactification of ω_{1} is ω_{1}+1, just as its one-point compactification (in sharp contrast to ω, whose Stone–Čech compactification is much *larger* than ω).

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*. This concept, a **transfinite sequence** or **ordinal-indexed sequence**, is a generalization of the concept of a sequence. An ordinary sequence corresponds to the case *α* = ω.

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 *x*_{ι} is in *U* for all *ι*≥*β*.

Ordinal-indexed sequences are more powerful than ordinary (ω-indexed) sequences to determine limits in topology: for example, ω_{1} (omega-one, the set of all countable ordinal numbers, and the smallest uncountable ordinal number), is a limit point of ω_{1}+1 (because it is a limit ordinal), and, indeed, it is the limit of the ω_{1}-indexed sequence which maps any ordinal less than ω_{1} to itself: however, it is not the limit of any ordinary (ω-indexed) sequence in ω_{1}, since any such limit is less than or equal to the union of its elements, which is a countable union of countable sets, hence itself countable.