History of the Parallel Postulate Chapter 5
Clavius’ Axiom For any line l and any point P not on l, the equidistant locus to l through P is the set of all the points on a line through P.
Theorem; The following three statements are equivalent for a Hilbert plane: a) The plane is semi-Euclidean b) For any line l and any point P not on l the equidistant locus to l through P is the set of all the points on the parallel to l through P obtained by the standard construction. c) Clavis’ axiom.
Wallis Postulate: Given any triangle ABC and given any segment DE, there exists a triangle DEF having DE as one of its sides such that ABC ~ DEF. (Two triangles are similar ( ~) if their vertices can be put in 1-1 correspon- dence in such a way that corresponding angles are congruent.
Clairaut’s Axiom Rectangles exist.
Proclus Theorem: The Euclidean parallel postulate hold on a Hilbert plane iff the plane is semi-Euclidean (i.e., the angle sum of a triangle is 180°) and Aristotle’s angle unboundedness axiom holds. In particular, the Euclidean parallel postulate holds in an Archimedean semi-Euclidean plane.
Legendre ( with Archimedes Axiom) Theorem: For any acute A and any point D in the interior of A, there exists a line through D and not through A that interesects both sides of A. (The sum of every triangle is 180°) Axiom: For any acute angle and any point in the interior of that angle, there exists a line through that point and not through the angle vertex which intersects both sides of the angle.