In mathematics, a non-Desarguesian plane, named after Girard Desargues, is a projective plane that does not satisfy Desargues' theorem, or in other words a plane that is not a Desarguesian plane. The theorem of Desargues is valid in all projective spaces of dimension not 2, that is, all the classical projective geometries over a field (or division ring), but Hilbert found that some projective planes do not satisfy it. Understanding of these examples is not complete, in the current state of knowledge.
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Examples
Several examples are also finite. For a finite projective plane, the order is one less than the number of points on a line (a constant for every line). Some of the known examples of non-Desarguesian planes include:
Classification
According to Weibel (2007, pg. 1296), H. Lenz gave a classification scheme for projective planes in 1954 and this was refined by A. Barlotti in 1957. This classification scheme is based on the types of point–line transtitivity permitted by the collineation group of the plane and is known as the Lenz–Barlotti classification of projective planes. The list of 53 types is given in Dembowski (1968, pp.124–5) and a table of the then known existence results (for both collineation groups and planes having such a collineation group) in both the finite and infinite cases appears on page 126. According to Weibel "36 of them exist as finite groups. Between 7 and 12 exist as finite projective planes, and either 14 or 15 exist as infinite projective planes."
Other classification schemes exist. One of the simplest is based on the type of planar ternary ring (PTR) which can be used to coordinatize the projective plane. The types are fields, skewfields, alternative division rings, semifields, nearfields, right nearfields, quasifields and right quasifields.
Conics
In a Desarguesian projective plane a conic can be defined in several different ways that can be proved to be equivalent. In non-Desarguesian planes these proofs are no longer valid and the different definitions can give rise to non-equivalent objects. T.G. Ostrom had suggested the name conicoid for these conic-like figures but did not provide a formal definition and the term does not seem to be widely used.
There are several ways that conics can be defined in Desarguesian planes:
- The set of absolute points of a polarity is known as a von Staudt conic. If the plane is defined over a field of characteristic two, only degenerate conics are obtained.
- The set of points of intersection of corresponding lines of two pencils which are projectively, but not perspectively, related is known as a Steiner conic. If the pencils are perspectively related, the conic is degenerate.
- The set of points whose coordinates satisfy an irreducible homogeneous equation of degree two.
Furthermore, in a finite Desarguesian plane:
- The set of q + 1 points, no three collinear in PG(2,q) is called an oval. If q is odd, an oval is a conic – in sense 3 above.
- An Ostrom conic (defined below) based on a generalization of harmonic sets.
Artzy has given an example of a Steiner conic in a Moufang plane which is not a von Staudt conic. Garner gives an example of a von Staudt conic that is not an Ostrom conic in a finite semifield plane.