Ideal point
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In hyperbolic geometry, an ideal point, omega point[1] or point at infinity is a well defined point outside the hyperbolic plane or space. Given a line l and a point P not on l, right- and left-limiting parallels to l through P converge to l at ideal points.
Unlike the projective case, ideal points form a boundary, not a submanifold. So, these lines do not intersect at an ideal point and such points, although well defined, do not belong to the hyperbolic space itself.
The ideal points together form the Cayley absolute or boundary of a hyperbolic geometry. For instance, the unit circle forms the Cayley absolute of the Poincaré disk model and the Klein disk model. While the real line forms the Cayley absolute of the Poincaré half-plane model .[2]
Pasch's axiom and the exterior angle theorem still hold for an omega triangle, defined by two points in hyperbolic space and an omega point.[3]
Contents
Properties
- The hyperbolic distance between an ideal point and any other point or ideal point is infinite.
- The centres of horocycles and horoballs are ideal points; two horocycles are concentric when they have the same centre.
Ideal triangles
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if all vertices of a triangle are ideal points the triangle is an ideal triangle.
Ideal triangles have a number of interesting properties:
- All ideal triangles are congruent.
- The interior angles of an ideal triangle are all zero.
- Any ideal triangle has area
where K is the (negative) curvature of the plane.[4]
Ideal n- polygons
As n- polygons can be subdivided in (n-2) ideal triangles their area is (n-2) times the area of an ideal triangle.
Representations in models of hyperbolic geometry
Klein disk model
In the Klein disk model and the Poincaré disk model of the hyperbolic plane. In both disk models the ideal points are on the unit circle (hyperbolic plane) or unit sphere (higher dimensions) which is the unreachable boundary of the hyperbolic plane.
When projecting the same hyperbolic line to the Klein disk model and the Poincaré disk model both lines go through the same two ideal points.(the ideal points in both models are on the same spot).
Given two distinct points p and q in the open unit disk the unique straight line connecting them intersects the unit circle in two ideal points, a and b, labeled so that the points are, in order, a, p, q, b so that |aq| > |ap| and |pb| > |qb|. Then the hyperbolic distance between p and q is expressed as
Poincaré disk model
In the Klein disk model and the Poincaré disk model of the hyperbolic plane. In both disk models the ideal points are on the unit circle which is the unreachable boundary of the hyperbolic plane.
When projecting the same hyperbolic line to the Klein disk model and the Poincaré disk model both lines go through the same two ideal points.(the ideal points in both models are on the same spot).
Given two distinct points p and q in the open unit disk then the unique circle arc orthogonal to the boundary connecting them intersects the unit circle in two ideal points, a and b, labeled so that the points are, in order, a, p, q, b so that |aq| > |ap| and |pb| > |qb|.
Then the hyperbolic distance between p and q is expressed as
Where the distances are measured along the (straight line) segments aq, ap, pb and qb.
Poincaré half-plane model
In the Poincaré half-plane model the ideal points are the points on the boundary axis and also the euclidean line at is an ideal point.
Hyperboloid model
In the hyperboloid model there are no ideal points.
See also
- ideal triangle
- Points at infinity for uses in other geometries.