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 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.
The standard topologies on R, Q, Z, and N are the order topologies.
Induced order topology
If Y is a subset of X, 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.For example, consider the subset Y = ∪ n∈N in the rationals. Under the subspace topology, the singleton set 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.
An example of a subspace of a linearly ordered space whose topology is not an order topology
Though the subspace topology of Y = ∪ 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, so the subspace topology is the discrete topology on Y, 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 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 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 strict total order < on Z such that the order topology generated by < is equal to the subspace topology on Z. In the following, interval notation should be interpreted relative to the < relation. Also, if A and B are sets, shall mean that for each a in A and b in B.
Let M = Z \ , the unit interval. M is connected. If m, n ∈ M and m < -1 < n, then and separate M, a contradiction. Thus, M < or < M. Assume without loss of generality that < M. Since is open in Z, there is some point p in M such that the interval is empty. Since < M, we know -1 is the only element of Z that is less than p, so p is the minimum of M. Then M \ = A ∪ B, where A and B are nonempty 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. Assume without loss of generality that A < B. If a is any point in A, then p < a and A. Then, so p,a) is open. ∪A=[p,a)∪A, so ∪A is an open subset of M and hence M = ∪ B is the union of two disjoint open subsets of M so M is not connected, a contradiction.
[Left and right
order topologiesSeveral variants of the order topology can be given:- The right order topology on X is the topology whose open sets consist of intervals of the form .
- The left order topology on X is the topology whose open sets consist of intervals of the form .
The left order topology is the standard topology used for many set-theoretic purposes on a Boolean algebra.
Ordinal space
For any ordinal number λ one can consider the spaces of ordinal numberstogether with the natural order topology. These spaces are called ordinal spaces.. Obviously, these spaces are mostly of interest when λ is an infinite ordinal; otherwise, the order topology is simply the discrete topology.
When λ = ω is just N with the usual topology, while is the one-point compactification of N.
Of particular interest is the case when λ = ω1, the set of all countable ordinals, and the first uncountable ordinal. The element ω1 is a limit point of the subset is not first-countable. The subspace is separable or second-countable
- is compact while sequentially compact and countably compact, but not compact or [paracompact">Sequentially compact space">sequentially compact and countably compact, but not compact or [paracompact
Topology and ordinals
Ordinals as topological spaces
Any ordinal number can be made into a topological space by endowing it with the order topology : 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.The set of limit points of an ordinal α is precisely the set of limit ordinals less than α. Successor ordinals 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, scattered, zero-dimensional as the union of the clopen intervals. However, they are not extremally disconnected in general.
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: an element in ω1 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 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 is eventually constant: so the Stone–Čech compactification of ω1 is ω1+1, just as its one-point compactification.
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. 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 xι is in U for all ι≥β.
Ordinal-indexed sequences are more powerful than ordinary sequences to determine limits in topology: for example, ω1, is a limit point of ω1+1, 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 sequence in ω1, 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.
However, ordinal-indexed sequences are not powerful enough to replace nets in general: for example, on the Tychonoff plank, the corner point is a limit point of the open subset, but it is not the limit of an ordinal-indexed sequence.