# Homepage of Boris Haase

## #54: Shortening and Improvement Euclidean Geometry on 24.11.2013

Definition: Two distinct points x and z in a Euclidean space (simply called a space in the following) viewed as a subspace of ℝn with n ∈ ωℕ* (see Set Theory) are said to be a pair (of points). A line segment is a pair (x, z) together with all inner points y in the space that are distinct from both the starting point x and the end point z and that lie between x and z, satisfying ||x - y|| + ||y - z|| = ||x - z|| with respect to the Euclidean norm || ||.

Definition: Two line segments are said to intersect if they have precisely one point in common. This includes the case when the common point is only found after completing each line segment with all other inner points of that segment within ℝn. A (one-dimensional) set of points in the space with the property that each point has at least one and at most two gaplessly neighbouring points is called a line, and a maximal two-dimensional subspace is called a plane.

Definition: A line segment is said to be a straight line if both its starting point and its end point lie on the boundary of the space, for the time being with the additional requirement that none of its inner points do. Two line segments are said to be parallel if the minimum distances between each point on one line segment and the other line segment are identical, and the same is true for all sub-line segments. Any line segment in the space parallel to one of the straight lines defined above is also called a straight line.

Definition: Two lines in a plane are said to be parallel if every point of one line has the same distance from the other line and vice versa, or if one line may be obtained from the other by means of a translation. Two planes in the space are said to be parallel is they are contained in planes of ℝn that may be obtained from each other by means of a translation.

Result: By defining short straight lines, arbitrarily many counterexamples can be given based on the above to Pasch's axiom, the axiom of line completeness, as well as various other axioms and their equivalents. The parallel postulate is redundant in Euclidean Geometry, since the parallel straight line running through a given point is uniquely defined by the shortest distance from the original straight line.

If two straight lines are only considered to be parallel when they lie in the same plane and do not intersect, then the parallel postulate does not hold when the reciprocal of the distance between the straight line and the given point is greater than infinity or smaller than |ωℕ|, since then infinitely many distinct straight lines can be found that pass through the given point without intersecting the original straight line.

The Archimedean axiom must be extended to the case where a segment is marked off an infinite natural number of times without exceeding the starting point or end point of a straight line, or replaced by the Archimedean theorem (in the finite case). Pasch's axiom is also unnecessary, since every straight line must be fully contained in the interior of some triangle due to its maximum length, and hence so must its boundary.