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In mathematics, hyperbolic n-space, denoted Hⁿ, is the maximally symmetric, simply connected, n-dimensional Riemannian manifold with constant sectional curvature −1. Hyperbolic space is the principal example of a space exhibiting hyperbolic geometry. It can be thought of as the negative-curvature analogue of the n-sphere. Although hyperbolic space Hⁿ is diffeomorphic to Rⁿ its negative-curvature metric gives it very different geometric properties.

Hyperbolic 2-space, H², is also called the hyperbolic plane.

Motivation edit

The concept of curvature may be intuitively clear for a curve. In order to understand curved surfaces; however, it is useful to build simple ones. A nice flat or zero curvature surface is the plane R². And a nice positively curved surface is the sphere S². Both the plane and the sphere are the same at every point, a property called homogeneity. By comparing these we gain an intuition for positive curvature, and may begin wondering if (for example) all positively curved surfaces are bounded like the sphere.

In order to understand negatively curved surfaces we make a simple one that is homogeneous. This leads to the hyperbolic space H². While the sphere is the set of points solving the equation $x^{2}+y^{2}+z^{2}=1$ , hyperbolic surfaces are (except for one annoying detail) the set of points solving $x^{2}-y^{2}-z^{2}=1$ . The annoying detail is that this equation actually describes two separate surfaces that are each hyperbolic spaces. If we solve the equation for x we find that either $x=(1+y^{2}+z^{2})^{1/2}$ or else $x=-(1+y^{2}+z^{2})^{1/2}$ . Both are hyperbolic spaces: homogeneous with negative curvature.

Models of hyperbolic space edit

Hyperbolic space, developed independently by Lobachevsky and Bolyai, is a geometrical space analogous to Euclidean space, but such that Euclid's parallel postulate is no longer assumed to hold. Instead, the parallel postulate is replaced by the following alternative (in two dimensions):

Given any line L and point P not on L, there are at least two distinct lines passing through P which do not intersect L.

It is then a theorem that there are in fact infinitely many such lines through P. Note that this axiom still does not uniquely characterize the hyperbolic plane uniquely up to isometry; there is an extra constant, the curvature K < 0, which must be specified. However, it does uniquely characterize it up to homothety, meaning up to bijections which only change the notion of distance by an overall constant. By choosing an appropriate length-scale, one can thus assume, without loss of generality, that K=-1.

Hyperbolic spaces are constructed in order to model such a modification of Euclidean geometry. In particular, the existence of model spaces implies that the parallel postulate is logically independent of the other axioms of Euclidean geometry.

There are several important models of hyperbolic space: the Klein model, the hyperboloid model, and the Poincaré model. These all model the same geometry in the sense that any two of them can be related by a transformation which preserves all the geometrical properties of the space. They are isometric.

The hyperboloid model edit

The first model realizes hyperbolic space as a hyperboloid in Rⁿ⁺¹ = {(x₀,...,x_n)|x_i∈R, i=0,1,...,n}. The hyperboloid is the locus Hⁿ of points whose coordinates satisfy

x_{0}^{2}-x_{1}^{2}-\ldots -x_{n}^{2}=1,\quad x_{0}>0.

In this model a "line" (or geodesic) is the curve cut out by intersecting Hⁿ with a plane through the origin in Rⁿ⁺¹.

The hyperboloid model is closely related to the geometry of Minkowski space. The quadratic form

Q(x)=x_{0}^{2}-x_{1}^{2}-x_{2}^{2}-\cdots -x_{n}^{2}

which defines the hyperboloid polarizes to give the bilinear form B defined by

B(x,y)=(Q(x+y)-Q(x)-Q(y))/2=x_{0}y_{0}-x_{1}y_{1}-\cdots -x_{n}y_{n}.

The space Rⁿ⁺¹, equipped with the bilinear form B is an (n+1)-dimensional Minkowski space R^n,1.

From this perspective, one can associate a notion of distance to the hyperboloid model, by defining^[1] the distance between two points x and y on H to be

d(x,y)=\operatorname {arccosh} \,B(x,y).

This function satisfies the axioms of a metric space. Moreover, it is preserved by the action of the Lorentz group on R^n,1. Hence the Lorentz group acts as a transformation group of isometries on Hⁿ.

The Klein model edit

An alternative model of hyperbolic geometry is on a certain domain in projective space. The Minkowski quadratic form Q defines a subset Uⁿ ⊂ RPⁿ given as the locus of points for which Q(x) > 0 in the homogeneous coordinates x. The domain Uⁿ is the Klein model of hyperbolic space.

The lines of this model are the open line segments of the ambient projective space which lie in Uⁿ. The distance between two points x and y in Uⁿ is defined by

d(x,y)=\operatorname {arccosh} \left({\frac {B(x,y)}{\sqrt {Q(x)Q(y)}}}\right).

Note that this is well-defined on projective space, since the ratio under the inverse hyperbolic cosine is homogeneous of degree 0.

This model is related to the hyperboloid model as follows. Each point x ∈ Uⁿ corresponds to a line L_x through the origin in Rⁿ⁺¹, by the definition of projective space. This line intersects the hyperboloid Hⁿ in a unique point. Conversely, through any point on Hⁿ, there passes a unique line through the origin (which is a point in the projective space). This correspondence defines a bijection between Uⁿ and Hⁿ. It is an isometry since evaluating d(x,y) along Q(x) = Q(y) = 1 reproduces the definition of the distance given for the hyperboloid model.

The Poincaré models edit

Main articles: Poincaré disc model, Poincaré half-plane model

Another closely related pair of models of hyperbolic geometry are the Poincaré ball and Poincaré half-space models. The ball model comes from a stereographic projection of the hyperboloid in Rⁿ⁺¹ onto the hyperplane {x₀ = 0}. In detail, let S be the point in R^n,1 with coordinates (-1,0,0,...,0): the South pole for the stereographic projection. For each point P on the hyperboloid Hⁿ, let P^* be the unique point of intersection of the line SP with the plane {x₀ = 0}. This establishes a bijective mapping of Hⁿ into the unit ball

B^{n}=\{(x_{1},\ldots ,x_{n})|x_{1}^{2}+\ldots +x_{n}^{2}<1\}

in the plane {x₀ = 0}.

The geodesics in this model are semicircles which are perpendicular to the boundary sphere of B_n. Isometries of the ball are generated by spherical inversion in hyperspheres perpendicular to the boundary.

The half-space model results from applying an inversion in a point of the boundary of Bⁿ. This sends circles to circles and lines, and is moreover a conformal transformation. Consequently the geodesics of the half-space model are lines and circles perpendicular to the boundary hyperplane.

Hyperbolic manifolds edit

Every complete, connected, simply-connected manifold of constant negative curvature −1 is isometric to the real hyperbolic space Hⁿ. As a result, the universal cover of any closed manifold M of constant negative curvature −1, which is to say, a hyperbolic manifold, is Hⁿ. Thus, every such M can be written as Hⁿ/Γ where Γ is a torsion-free discrete group of isometries on Hⁿ. That is, Γ is a lattice in SO⁺(n,1).

Riemann surfaces edit

Two-dimensional hyperbolic surfaces can also be understood according to the language of Riemann surfaces. According to the uniformization theorem, every Riemann surface is either elliptic, parabolic or hyperbolic. Most hyperbolic surfaces have a non-trivial fundamental group $\pi _{1}=\Gamma$ ; the groups that arise this way are known as Fuchsian groups. The quotient space H/Γ of the upper half-plane modulo the fundamental group is known as the Fuchsian model of the hyperbolic surface. The Poincaré half plane is also hyperbolic, but is simply connected and noncompact. It is the universal cover of the other hyperbolic surfaces.

The analogous construction for three-dimensional hyperbolic surfaces is the Kleinian model.

References edit

^ Note the similarity with the chordal metric on a sphere, which uses trigonometric instead of hyperbolic functions.

Ratcliffe, John G., Foundations of hyperbolic manifolds, New York, Berlin. Springer-Verlag, 1994.
Reynolds, William F. (1993) "Hyperbolic Geometry on a Hyperboloid", American Mathematical Monthly 100:442-455.
Wolf, Joseph A. Spaces of constant curvature, 1967. See page 67.

Category:Homogeneous spaces Category:Hyperbolic geometry Category:Topological spaces

[1] Note the similarity with the chordal metric on a sphere, which uses trigonometric instead of hyperbolic functions.

[1]