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 ideal endpoints on the boundary sphere.

The Beltrami–Klein model is named after the Italian geometer Eugenio Beltrami and the German Felix Klein while "Cayley" in Cayley–Klein model refers to the English geometer Arthur Cayley.

The Beltrami–Klein model is analogous to the gnomonic projection of spherical geometry, in that geodesics (great circles in spherical geometry) are mapped to straight lines.

This model is not conformal, meaning that angles and circles are distorted, whereas the Poincaré disk model preserves these.

In this model, lines and segments are straight Euclidean segments, whereas in the Poincaré disk model, lines are arcs that meet the boundary orthogonally.

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

The papers of Beltrami remained little noticed until recently and the model was named after Klein ("The Klein disk model"). This happened as follows. In 1859 Arthur Cayley used the cross-ratio definition of angle due to Laguerre to show how Euclidean geometry could be defined using projective geometry.^[4] His definition of distance later became known as the Cayley metric.

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

"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."^[5]

Later, Felix Klein realized that Cayley's ideas give rise to a projective model of the non-Euclidean plane.^[6]

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.^[7]

The distance function for the Beltrami–Klein model is a Cayley–Klein metric. Given two distinct points p and q in the open unit ball, the unique straight line connecting them intersects the boundary at two ideal points, a and b, label them so that the points are, in order, a, p, q, b , so that |aq| > |ap| and |pb| > |qb|.

The hyperbolic distance between p and q is then: ${\displaystyle d(p,q)={\frac {1}{2}}\ln {\frac {\left|aq\right|\,\left|pb\right|}{\left|ap\right|\,\left|qb\right|}}}$

The vertical bars indicate Euclidean distances between the points in the model, where ln is the natural logarithm and the factor of one half is needed to give the model the standard curvature of −1.

When one of the points is the origin and Euclidean distance between the points is r then the hyperbolic distance is:

${\displaystyle {\frac {1}{2}}\ln \left({\frac {1+r}{1-r}}\right)=\operatorname {artanh} r,}$

where artanh is the inverse hyperbolic function of the hyperbolic tangent.

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 (also called the absolute). The points on the boundary circle are called ideal points; although well defined, they do not belong to the hyperbolic plane. Neither do 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 distorted, as are horocycles and hypercycles

Given two distinct points U and V in the open unit ball of the model in Euclidean space, the unique straight line connecting them intersects the unit sphere at two ideal points A and B, labeled so that the points are, in order along the line, A, U, V, B. Taking the centre of the unit ball of the model as the origin, and assigning position vectors u, v, a, b respectively to the points U, V, A, B, we have that that ‖a − v‖ > ‖a − u‖ and ‖u − b‖ > ‖v − b‖, where ‖ · ‖ denotes the Euclidean norm. Then the distance between U and V in the modelled hyperbolic space is expressed as

${\displaystyle d(\mathbf {u} ,\mathbf {v} )={\frac {1}{2}}\ln {\frac {\left\|\mathbf {v} -\mathbf {a} \right\|\,\left\|\mathbf {b} -\mathbf {u} \right\|}{\left\|\mathbf {u} -\mathbf {a} \right\|\,\left\|\mathbf {b} -\mathbf {v} \right\|}},}$

where the factor of one half is needed to make the curvature −1.

The associated metric tensor is given by^[12][13]

${\displaystyle ds^{2}=g(\mathbf {x} ,d\mathbf {x} )={\frac {\left\|d\mathbf {x} \right\|^{2}}{1-\left\|\mathbf {x} \right\|^{2}}}+{\frac {(\mathbf {x} \cdot d\mathbf {x} )^{2}}{{\bigl (}1-\left\|\mathbf {x} \right\|^{2}{\bigr )}^{2}}}={\frac {(1-\left\|\mathbf {x} \right\|^{2})\left\|d\mathbf {x} \right\|^{2}+(\mathbf {x} \cdot d\mathbf {x} )^{2}}{{\bigl (}1-\left\|\mathbf {x} \right\|^{2}{\bigr )}^{2}}}}$

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

${\displaystyle \mathbf {x} \cdot \mathbf {y} =x_{0}y_{0}-x_{1}y_{1}-\cdots -x_{n}y_{n}\,}$

and the norm by ${\displaystyle \left\|\mathbf {x} \right\|={\sqrt {\mathbf {x} \cdot \mathbf {x} }}}$. 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

${\displaystyle d(\mathbf {u} ,\mathbf {v} )=\operatorname {arcosh} (\mathbf {u} \cdot \mathbf {v} ).}$

This may also be written in the homogeneous form

${\displaystyle d(\mathbf {u} ,\mathbf {v} )=\operatorname {arcosh} \left({\frac {\mathbf {u} }{\left\|\mathbf {u} \right\|}}\cdot {\frac {\mathbf {v} }{\left\|\mathbf {v} \right\|}}\right),}$

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.

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 ${\displaystyle u}$ 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

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

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

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

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.

Wikimedia Commons has media related to Beltrami–Klein models.

• Poincaré half-plane model
• Poincaré disk model
• Poincaré metric
• Inversive geometry