non-Euclidean geometry


A non-Euclidean geometry is a in which at least one of the axioms from Euclidean geometryMathworldPlanetmath fails. Within this entry, only geometriesMathworldPlanetmath that are considered to be two-dimensional will be considered.

The most common non-Euclidean geometries are those in which the parallel postulateMathworldPlanetmath fails; i.e. (http://planetmath.org/Ie), there is not a unique line that does not intersect a given line through a point not on the given line. Note that this is equivalentMathworldPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath to saying that the sum of the angles of a triangleMathworldPlanetmath is not equal to π radians.

If there is more than one such parallel lineMathworldPlanetmath, the is called hyperbolic (or Bolyai-Lobachevski). In these of , the sum of the angles of a triangle is strictly in 0 and π radians. (This sum is not constant as in Euclidean geometry; it depends on the area of the triangle. See the entry regarding defect for more details.)

As an example, consider the disc {(x,y)2:x2+y2<1} in which a point is similarMathworldPlanetmath to the EuclideanMathworldPlanetmathPlanetmath point and a line is defined to be a chord (excluding its endpointsMathworldPlanetmath) of the (circular (http://planetmath.org/Circle)) boundary. This is the Beltrami-Klein model for 2. It is relatively easy to see that, in this , given a line and a point not on the line, there are infinitely many lines passing through the point that are parallelMathworldPlanetmath to the given line.

If there is no parallel line, the is called spherical (or elliptic). In these of , the sum of the angles of a triangle is strictly in π and 3π radians. (This sum is not constant as in Euclidean geometry; it depends on the area of the triangle. See the entries regarding defect (http://planetmath.org/Defect) and area of a spherical triangle for more details.)

As an example, consider the surface of the unit sphereMathworldPlanetmath (http://planetmath.org/Sphere) {(x,y,z)3:x2+y2+z2=1} in which a point is similar to the Euclidean point and a line is defined to be a great circle. (Note that, when a sphere serves as a model of spherical geometry, its radius is typically assumed to be 1.) It is relatively easy to see that, in this , given a line and a point not on the line, it is impossible to find a line passing through the point that does not intersect the given line.

Note also that, in spherical geometry, two distinct points do not necessarily determine a unique line; however, two distinct points that are not antipodal always determine a unique line.

One final example of a non-Euclidean is semi-Euclidean geometry, in which the axiom of Archimedes fails.

Title non-Euclidean geometry
Canonical name NonEuclideanGeometry
Date of creation 2013-03-22 13:54:51
Last modified on 2013-03-22 13:54:51
Owner Wkbj79 (1863)
Last modified by Wkbj79 (1863)
Numerical id 22
Author Wkbj79 (1863)
Entry type Definition
Classification msc 51-00
Classification msc 51M10
Related topic Sphere
Related topic ComparisonOfCommonGeometries
Defines hyperbolic geometry
Defines Bolyai-Lobachevski geometry
Defines elliptic geometry
Defines spherical geometry
Defines semi-Euclidean geometry