Intopology, thelong line (orAlexandroff line) is atopological space somewhat similar to thereal line, but in a certain sense "longer". It behaves locally just like the real line, but has different large-scale properties (e.g., it is neitherLindelöf norseparable). Therefore, it serves as an importantcounterexample in topology.^[1] Intuitively, the usual real-number line consists of a countable number of line segments${\displaystyle [0,1)}$ laid end-to-end, whereas the long line is constructed from an uncountable number of such segments.

Theclosed long ray${\displaystyle L}$ is defined as theCartesian product of thefirst uncountable ordinal${\displaystyle {\omega }_1}$ with thehalf-open interval${\displaystyle [0,1),}$ equipped with theorder topology that arises from thelexicographical order on${\displaystyle {\omega }_1\times [0,1)}$. Theopen long ray is obtained from the closed long ray by removing the smallest element${\displaystyle (0,0).}$

Thelong line is obtained by "gluing" together two long rays, one in the positive direction and the other in the negative direction. More rigorously, it can be defined as theorder topology on the disjoint union of the reversed open long ray (“reversed” means the order is reversed) (this is the negative half) and the (not reversed) closed long ray (the positive half), totally ordered by letting the points of the latter be greater than the points of the former. Alternatively, take two copies of the open long ray and identify the open interval${\displaystyle \{0\}\times (0,1)}$ of the one with the same interval of the other but reversing the interval, that is, identify the point${\displaystyle (0,t)}$ (where${\displaystyle t}$ is a real number such that) of the one with the point${\displaystyle (0,1-t)}$ of the other, and define the long line to be the topological space obtained by gluing the two open long rays along the open interval identified between the two. (The former construction is better in the sense that it defines the order on the long line and shows that the topology is the order topology; the latter is better in the sense that it uses gluing along an open set, which is clearer from the topological point of view.)

Intuitively, the closed long ray is like a real (closed) half-line, except that it is much longer in one direction: we say that it is long at one end and closed at the other. The open long ray is like the real line (or equivalently an open half-line) except that it is much longer in one direction: we say that it is long at one end and short (open) at the other. The long line is longer than the real lines in both directions: we say that it is long in both directions.

However, many authors speak of the “long line” where we have spoken of the (closed or open) long ray, and there is much confusion between the various long spaces. In many uses or counterexamples, however, the distinction is unessential, because the important part is the “long” end of the line, and it doesn't matter what happens at the other end (whether long, short, or closed).

A related space, the (closed)extended long ray,${\displaystyle L^{\ast },}$ is obtained as theone-point compactification of${\displaystyle L}$ by adjoining an additional element to the right end of${\displaystyle L.}$ One can similarly define theextended long line by adding two elements to the long line, one at each end.

The closed long ray${\displaystyle L={\omega }_1\times [0,1)}$ consists of an uncountable number of copies of${\displaystyle [0,1)}$ 'pasted together' end-to-end. Compare this with the fact that for anycountableordinal${\displaystyle \alpha }$, pasting together${\displaystyle \alpha }$ copies of${\displaystyle [0,1)}$ gives a space which is still homeomorphic (and order-isomorphic) to${\displaystyle [0,1).}$ (And if we tried to glue togethermore than${\displaystyle {\omega }_1}$ copies of${\displaystyle [0,1),}$ the resulting space would no longer be locally homeomorphic to${\displaystyle \mathbb{R}.}$)

Every increasingsequence in${\displaystyle L}$ converges to alimit in${\displaystyle L}$; this is a consequence of the facts that (1) the elements of${\displaystyle {\omega }_1}$ are thecountable ordinals, (2) thesupremum of every countable family of countable ordinals is a countable ordinal, and (3) every increasing and bounded sequence of real numbers converges.Consequently, there can be no strictly increasing function${\displaystyle L\to \mathbb{R}.}$ In fact, every continuous function${\displaystyle L\to \mathbb{R}}$ is eventually constant.

As order topologies, the (possibly extended) long rays and lines arenormalHausdorff spaces. All of them have the samecardinality as the real line, yet they are 'much longer'.All of them arelocally compact. None of them ismetrizable; this can be seen as the long ray issequentially compact but notcompact, nor evenLindelöf.

The (non-extended) long line or ray is notparacompact. It ispath-connected,locally path-connected andsimply connected but notcontractible. It is a one-dimensional topologicalmanifold, with boundary in the case of the closed ray. It isfirst-countable but notsecond countable and notseparable, so authors who require the latter properties in their manifolds do not call the long line a manifold.^[2]

It makes sense to consider all the long spaces at once because every connected (non-empty) one-dimensional (not necessarilyseparable)topological manifold possibly with boundary, ishomeomorphic to either the circle, the closed interval, the open interval (real line), the half-open interval, the closed long ray, the open long ray, or the long line.^[3]

The long line or ray can be equipped with the structure of a (non-separable)differentiable manifold (with boundary in the case of the closed ray). However, contrary to the topological structure which is unique (topologically, there is only one way to make the real line "longer" at either end), the differentiable structure is not unique:in fact, there are uncountably many (${\displaystyle 2^{{\mathrm{\aleph }}_1}}$ to be precise) pairwise non-diffeomorphic smooth structures on it.^[4] This is in sharp contrast to the real line, where there are also different smooth structures, but all of them are diffeomorphic to the standard one.

The long line or ray can even be equipped with the structure of a (real)analytic manifold (with boundary in the case of the closed ray). However, this is much more difficult than for the differentiable case (it depends on the classification of (separable) one-dimensional analytic manifolds, which is more difficult than for differentiable manifolds). Again, any given${\displaystyle C^{\mathrm{\infty }}}$ structure can be extended in infinitely many ways to different${\displaystyle C^{\omega }}$ (=analytic) structures (which are pairwise non-diffeomorphic as analytic manifolds).^[5]

The long line or ray cannot be equipped with aRiemannian metric that induces its topology.The reason is thatRiemannian manifolds, even without the assumption of paracompactness, can be shown to be metrizable.^[6]

The extended long ray${\displaystyle L^{\ast }}$ iscompact. It is the one-point compactification of the closed long ray${\displaystyle L,}$ but it isalso itsStone-Čech compactification, because anycontinuous function from the (closed or open) long ray to the real line is eventually constant.^[7]${\displaystyle L^{\ast }}$ is alsoconnected, but notpath-connected because the long line is 'too long' to be covered by a path, which is a continuous image of an interval.${\displaystyle L^{\ast }}$ is not a manifold and is not first countable.

There exists ap-adic analog of the long line, which is due toGeorge Bergman.^[8]

This space is constructed as the increasing union of an uncountable directed set of copies${\displaystyle X_{\gamma }}$ of the ring ofp-adic integers, indexed by a countable ordinal${\displaystyle \gamma .}$ Define a map from${\displaystyle X_{\delta }}$to${\displaystyle X_{\gamma }}$ whenever as follows:

• If${\displaystyle \gamma }$ is a successor${\displaystyle \epsilon +1}$ then the map from${\displaystyle X_{\epsilon }}$ to${\displaystyle X_{\gamma }}$ is just multiplication by${\displaystyle p.}$ For other${\displaystyle \delta }$ the map from${\displaystyle X_{\delta }}$ to${\displaystyle X_{\gamma }}$ is the composition of the map from${\displaystyle X_{\delta }}$ to${\displaystyle X_{\epsilon }}$ and the map from${\displaystyle X_{\epsilon }}$ to${\displaystyle X_{\gamma }.}$
• If${\displaystyle \gamma }$ is a limit ordinal then the direct limit of the sets${\displaystyle X_{\delta }}$ for is a countable union ofp-adic balls, so can be embedded in${\displaystyle X_{\gamma },}$ as${\displaystyle X_{\gamma }}$ with a point removed is also a countable union ofp-adic balls. This defines compatible embeddings of${\displaystyle X_{\delta }}$ into${\displaystyle X_{\gamma }}$ for all

This space is not compact, but the union of any countable set of compact subspaces has compact closure.

Some examples of non-paracompact manifolds in higher dimensions include thePrüfer manifold, products of any non-paracompact manifold with any non-empty manifold, the ball of long radius, and so on. Thebagpipe theorem shows that there are${\displaystyle 2^{{\mathrm{\aleph }}_1}}$ isomorphism classes of non-paracompact surfaces, even when a generalization of paracompactness,ω-boundedness, is assumed.

There are no complex analogues of the long line as everyRiemann surface is paracompact, but Calabi and Rosenlicht gave an example of a non-paracompact complex manifold of complex dimension 2.^[9]

• Lexicographic order topology on the unit square
• List of topologies

• Topological spaces

• Articles with short description
• Short description is different from Wikidata