# Beltrami–Klein model

Lines in the projective model of the hyperbolic plane.
A hyperbolic triheptagonal tiling in a Beltrami–Klein model projection
The regular hyperbolic dodecahedral honeycomb, {5,3,4}

In geometry, the Beltrami–Klein model, also called the projective model, Klein disk model, and the Cayley–Klein model, is a model of hyperbolic geometry in which points are represented by the points in the interior of the unit disk (or n-dimensional unit ball) and lines are represented by the chords, straight line segments with endpoints on the boundary sphere.

The Beltrami–Klein model is strongly analogous to the gnomonic projection of spherical geometry, which maps great circles to straight lines; the formulae relating these two to the hyperboloid model and the sphere, respectively, are very similar.

The major disadvantage of the model versus the Poincaré disk model is that it is not conformal, meaning that angles are distorted.

Given two distinct points p and q in the open unit ball, the unique straight line connecting them intersects the boundary at two points, a and b, labeled so that the points are, in order, a, p, q, b. The hyperbolic distance between p and q is then

$d(p,q)=\frac{1}{2} \log \frac{|qa||bp|}{|pa||bq|} ,$

where the vertical bars indicate Euclidean distances in the model.

## History

This model made its first appearance in two memoirs of Eugenio Beltrami published in 1868, first for dimension n = 2 and then for general n, devoted to showing equiconsistency of hyperbolic geometry with ordinary Euclidean geometry.[1][2][3]

Formally speaking, this model was constructed in 1859 by the English mathematician Arthur Cayley. Yet he considered it only as a construction in projective geometry and apparently did not notice the connection with non-Euclidean geometry.

In 1869, the young (twenty-year-old) Felix Klein became acquainted with Cayley's work. He recalled that in 1870, and gave a talk on the work of Cayley at the seminar of the famous mathematician Weierstrass, and, as he writes:

"I finished with a question whether there might exist a connection between the ideas of Cayley and Lobachevsky. I was given the answer that these two systems were conceptually widely separated."

As Klein puts it, "I allowed myself to be convinced by these objections and put aside this already mature idea." However, in 1871, he returned to this idea, formulated it mathematically, and published it.[4]

The distance is given by the Cayley–Klein metric and was first written down by Arthur Cayley in the context of projective and spherical geometry. Felix Klein recognized its importance for non-Euclidean geometry and popularized the subject.

## Distance formula

Arthur Cayley applied the cross-ratio from projective geometry to measurement of distances and angles in spherical geometry.[5] Later, Felix Klein realized that Cayley's ideas give rise to a projective model of the non-Euclidean plane.[6]

Given two distinct points p and q in the open unit ball, the unique straight line connecting them intersects the unit sphere in two points, a and b, labeled so that the points are, in order, a, p, q, b. Then the hyperbolic distance between p and q is expressed as

$d(p,q) = \frac{1}{2} \log \frac{ \left| qa \right| \left| bp \right| }{ \left| pa \right| \left| bq \right| } ,$

where the vertical bars indicate Euclidean distances. The factor of one half is needed to make the curvature −1.

The associated metric tensor is given by

$g (x, dx) = \frac{4 (x \cdot dx)^2}{(1 - \left\Vert x \right\Vert^2)^2} + \frac{4 \left\Vert dx \right\Vert^2}{(1 - \left\Vert x \right\Vert^2)}.$

## The Klein disk model

In two dimensions the Beltrami–Klein model is called the Klein disk model. It is a disk and the inside of the disk is a model of the entire hyperbolic plane. Lines in this model are represented by chords of the boundary circle, the points on the boundary circle are called ideal points. The boundary circle itself , sometimes called the absolute, is not a part of the model, and neither are points outside the disk, which are sometimes called ultra ideal points.

The model is not conformal, meaning that angles are distorted, and circles on the hyperbolic plane are in general not circular in the model. Only circles that have their centre at the centre of the boundary circle are not distorted. All other circles are, as are horocycles and hypercycles.

### Properties

Chords that meet on the boundary circle are limiting parallel lines.

Two chords that when extended beyond the model go through the pole of the other are mutually perpendicular, (The pole of a chord is an ultra ideal point – a point outside the disk) Chords that go through the centre of the disk have their pole at infinity, orthogonal to the direction of the chord (meaning that right angles on diameters are not distorted).

### Constructions

When one of the chords is a diameter of the boundary circle then the common perpendicular is the chord that is perpendicular to the diameter and that when lengthened goes through the pole of the other chord.

### Relation to the Poincaré disk model

the Beltrami–Klein model (line K), the Poincaré disk model (line P), and their relations with the other models

Both the Poincaré disk model and the Klein disk model are both models of the hyperbolic plane. An advantage of the Poincaré disk model is that it is conformal (circles and angles are not distorted); a disadvantage is that lines of the geometry are circular arcs orthogonal to the boundary circle of the disk.

The two models are related through a projection on or from the hemisphere model. The Klein model is an Orthographic projection to the hemisphere model while the Poincaré disk model is an stereographic projection.

When projecting the same lines in both models on one disk it will show that both lines go through the same two ideal points.(they remain on the same spot) also the pole of the chord is the centre of the circle that contains the arc.

If $u$ is a vector of norm less than one representing a point of the Poincaré disk model, then the corresponding point of the Beltrami–Klein model is given by:

$s = \frac{2u}{1+u \cdot u}.$

Conversely, from a vector $s$ of norm less than one representing a point of the Beltrami–Klein model, the corresponding point of the Poincaré disk model is given by:

$u = \frac{s}{1+\sqrt{1-s \cdot s}} = \frac{\left(1-\sqrt{1-s \cdot s}\right)s}{s \cdot s}.$

## Relation to the hyperboloid model

The hyperboloid model is a model of hyperbolic geometry within (n + 1)-dimensional Minkowski space. The Minkowski inner product is given by

$\mathbf{x} \cdot \mathbf{y} = x_0 y_0 - x_1 y_1 - \cdots - x_n y_n \,$

and the norm by $\left\| \mathbf{x} \right\| = \sqrt{ \mathbf{x} \cdot \mathbf{x} }$. The hyperbolic plane is embedded in this space as the vectors x with ||x|| = 1 and x0 (the "timelike component") positive. The intrinsic distance (in the embedding) between points u and v is then given by

$d ( \mathbf{u} , \mathbf{v} ) = \cosh^{-1} ( \mathbf{u} \cdot \mathbf{v}) .$

This may also be written in the homogeneous form

$d ( \mathbf{u} , \mathbf{v} ) = \cosh^{-1} \left( \frac{ \mathbf{u} }{ \left\| \mathbf{u} \right\| } \cdot \frac{ \mathbf{v} }{ \left\| \mathbf{v} \right\| } \right) ,$

which allows the vectors to be rescaled for convenience.

The Beltrami–Klein model is obtained from the hyperboloid model by rescaling all vectors so that the timelike component is 1, that is, by projecting the hyperboloid embedding through the origin onto the plane x0 = 1. The distance function, in its homogeneous form, is unchanged. Since the intrinsic lines (geodesics) of the hyperboloid model are the intersection of the embedding with planes through the Minkowski origin, the intrinsic lines of the Beltrami–Klein model are the chords of the sphere.

In the gyrovector space approach to hyperbolic geometry, vector algebra in the Beltrami–Klein model can be developed using relativistic 3-velocities as the vectors, analogously to the use of ordinary vectors in Euclidean geometry.

## Relation to the Poincaré ball model

Both the Poincaré ball model and the Beltrami–Klein model are models of the n-dimensional hyperbolic space in the n-dimensional unit ball in Rn. If $u$ is a vector of norm less than one representing a point of the Poincaré disk model, then the corresponding point of the Beltrami–Klein model is given by

$s = \frac{2u}{1+u \cdot u}.$

Conversely, from a vector $s$ of norm less than one representing a point of the Beltrami–Klein model, the corresponding point of the Poincaré disk model is given by

$u = \frac{s}{1+\sqrt{1-s \cdot s}} = \frac{\left(1-\sqrt{1-s \cdot s}\right)s}{s \cdot s}.$

Given two points on the boundary of the unit disk, which are traditionally called ideal points, the straight line connecting them in the Beltrami–Klein model is the chord between them, while in the corresponding Poincaré model the line is a circular arc on the two-dimensional subspace generated by the two boundary point vectors, meeting the boundary of the ball at right angles. The two models are related through a projection from the center of the disk; a ray from the center passing through a point of one model line passes through the corresponding point of the line in the other model.