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In mathematics, an order topology is a certain topology that can be defined on any totally ordered set. Mathematics is the body of Knowledge and Academic discipline that studies such concepts as Quantity, Structure, Space and Topology ( Greek topos, "place" and logos, "study" is the branch of Mathematics that studies the properties of In Mathematics and Set theory, a total order, linear order, simple order, or (non-strict ordering is a Binary relation It is a natural generalization of the topology of the real numbers to arbitrary totally ordered sets. In Mathematics, the real numbers may be described informally in several different ways

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. In Highway engineering, subbase is a layer between Subgrade and the Base course. This is equivalent to saying that the open intervals

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

together with the above rays form a base for the order topology. In Mathematics, a base (or basis) B for a Topological space X with topology T is a collection of Open sets The open sets in X are the sets that are a union of (possibly infinitely many) such open intervals and rays. In Set theory, the term Union (denoted as ∪ refers to a set operation used in the convergence of set elements to form a resultant set containing the elements of both sets

The order topology makes X into a completely normal Hausdorff space. In Topology and related branches of Mathematics, normal spaces, T4 spaces, T5 spaces, and T6 spaces In Topology and related branches of Mathematics, a Hausdorff space, separated space or T2 space is a Topological space

The standard topologies on R, Q, and N are the order topologies.

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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. In Topology and related areas of Mathematics, a subspace of a Topological space X is a Subset S of X which is 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. In Mathematics, a rational number is a number which can be expressed as a Ratio of two Integers Non-integer rational numbers (commonly called fractions 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.

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 strict 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). In Mathematics and Set theory, a total order, linear order, simple order, or (non-strict ordering is a Binary relation In the following, interval notation should be interpreted relative to the < relation. Also, if A and B are sets, A<B shall mean that a<b for each a in A and b in B.

Let M=Z\{-1}, the unit interval. M is connected. If m,n∈M and m<-1<n, then {k∈M:k<-1} and {k∈M:-1<k} separate M, a contradiction. Thus, M<{-1} or {-1}<M. Assume without loss of generality that {-1}<M. Since {-1} is open in Z, there is some point p in M such that the interval (-1,p) is empty. Since {-1}<M, -1 is the only element of Z that is less than p, so p is the minimum of M. Then M\{p} = A ∪ B, where A and B are open and disjoint connected subsets of M (removing a point from an open interval yields two open intervals). 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 (p,a)\subseteq A. Then (-1,a)=[p,a), so [p,a) is open. {p}∪A=[p,a)∪A, so {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.

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.

Left and right order topologies

Several variants of the order topology can be given:

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. In Abstract algebra, a Boolean algebra or Boolean lattice is a complemented distributive lattice.

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. In Set theory, an ordinal number, or just ordinal, is the Order type of a Well-ordered set. 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. In Topology, a discrete space is a particularly simple example of a Topological space or similar structure one in which the points are " isolated "

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 = ω1, the set of all countable ordinals, and 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 Mathematics, informally speaking a limit point of a set S in a Topological space X is a point x in X that can be "approximated" In particular, [0,ω1] is not first-countable. In Topology, a branch of Mathematics, a first-countable space is a Topological space satisfying the "first Axiom of countability " The subspace [0,ω1) is first-countable however, since the only point without a countable local base is ω1. In Topology and related areas of Mathematics, the neighbourhood system or neighbourhood filter \mathcal{V}(x for a point x is the Some further properties include

Topology and ordinals

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. In Mathematics a Topological space is called separable if it contains a countable dense subset that is there exists a sequence \{ x_n In Topology, a second-countable space is a Topological space satisfying the " second Axiom of countability " In Mathematics, a paracompact space is a Topological space in which every Open cover admits an open locally finite refinement. In Set theory, an ordinal number, or just ordinal, is the Order type of a Well-ordered set. Topological spaces are mathematical structures that allow the formal definition of concepts such as Convergence, connectedness, and continuity. In Mathematics and Set theory, a total order, linear order, simple order, or (non-strict ordering is a Binary relation (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. In Mathematics, informally speaking a limit point of a set S in a Topological space X is a point x in X that can be "approximated" A limit ordinal is an Ordinal number which is neither zero nor a Successor ordinal. Successor ordinals (and zero) less than α are isolated points in α. In Topology, a branch of Mathematics, a point x of a set S is called an isolated point,if there exists a neighborhood of In particular, the finite ordinals and ω are discrete topological spaces, and no ordinal beyond that is discrete. In Topology, a discrete space is a particularly simple example of a Topological space or similar structure one in which the points are " isolated " The ordinal α is compact as a topological space if and only if α is a successor ordinal. When defining the ordinal numbers, an absolutely fundamental operation that we can perform on them is a successor operation S to get the next higher one

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. In Mathematics, compactification is the process or result of enlarging a Topological space to make it compact. In Mathematics, the direct limit (also called the inductive limit) is a general method of taking colimits of "directed families of objects"

As topological spaces, all the ordinals are Hausdorff and even normal. In Topology and related branches of Mathematics, a Hausdorff space, separated space or T2 space is a Topological space In Topology and related branches of Mathematics, normal spaces, T4 spaces, T5 spaces, and T6 spaces 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). In Topology and related branches of Mathematics, a totally disconnected space is a Topological space which is maximally disconnected in the sense that In Mathematics, a Topological space is zero-dimensional or 0-dimensional, if its Topological dimension is zero or equivalently if it has a However, they are not extremally disconnected in general (there is an open set, namely ω, whose closure is not open). In Mathematics, a Topological space is termed extremally disconnected or extremely disconnected if the closure of every open set in it is 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: 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. In Topology, a branch of Mathematics, a first-countable space is a Topological space satisfying the "first Axiom of countability " In Topology, a second-countable space is a Topological space satisfying the " second Axiom of countability " 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). In Mathematics, the real line is simply the set R of singleton Real numbers However this term is usually used when R is to be treated as a In Mathematics, compactification is the process or result of enlarging a Topological space to make it compact.

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. In Mathematics, a sequence is an ordered list of objects (or events 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. This article is about nets in Topological spaces and not about ε-nets in Metric spaces In Topology and related areas of Mathematics

Ordinal-indexed sequences are more powerful than ordinary (ω-indexed) sequences to determine limits in topology: for example, ω1 (omega-onehttp://en.wikipedia.org../../../../articles/o/r/d/Ordinal_number.html#Initial_ordinal_of_a_cardinal, the set of all countable ordinal numbers, and the smallest uncountable ordinal number), 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 such limit is less than or equal to the union of its elements, which is a countable union of countable sets, hence itself countable. In Set theory, an ordinal number, or just ordinal, is the Order type of a Well-ordered set.

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. In Mathematics, a filter is a special Subset of a Partially ordered set. In Topology, the Tychonoff plank is a Topological space that is a Counterexample to several plausible-sounding Conjectures It is defined as

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References

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