Pappus axiom
If $l$ and $l'$ are two distinct straight lines and $A,B,C$ and $A',B',C'$ are distinct points on $l$ and $l'$, respectively, and if none of these is the point of intersection of $l$ and $l'$, then the points of intersection of $AB'$ and $A'B$, $BC'$ and $B'C$, $AC'$ and $A'C$ are collinear.
<img style="border:1px solid;" src="https://www.encyclopediaofmath.org/legacyimages/common_img/p071140a.gif" />
Figure: p071140a
The truth of Pappus' axiom is equivalent to the commutativity of the skew-field of the corresponding projective geometry. The Desargues assumption is a consequence of Pappus' axiom (Hessenberg's theorem), and at the same time Pappus' axiom is a degenerate case of the Pascal theorem. The axiom was proposed by Pappus (3rd century).
Comments
References
| [a1] | O. Veblen, J.W. Young, "Projective geometry" , 1 , Ginn (1910) |
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