Math 2.0

Geometry Definition

Moufang plane

Builds on Projective planeCollinear points

Definition

A projective plane is a Moufang plane if it satisfies the little Desargues property for every line ℓ\ell and every point OO on ℓ\ell:

  1. let ABCABC and A′B′C′A'B'C' be triangles with no vertex on ℓ\ell, with A≠A′A \ne A', B≠B′B \ne B', C≠C′C \ne C', and with AA′AA', BB′BB', CC′CC' through OO;
  2. suppose the points AB∩A′B′AB \cap A'B' and AC∩A′C′AC \cap A'C' lie on ℓ\ell;
  3. then the point BC∩B′C′BC \cap B'C' also lies on ℓ\ell.

Remarks

So three collinear intersection points are required only when the centre OO lies on the axis ℓ\ell. Equivalently, every line of the plane is a translation line, that is, the group of elations with axis ℓ\ell acts transitively on the points off ℓ\ell. A projective plane is a Moufang plane exactly when it can be coordinatized by an alternative division ring.

Example: ℙ2(k)\mathbb{P}^2(k) over a field kk is a Moufang plane. Check: Desargues's theorem holds in ℙ2(k)\mathbb{P}^2(k) for all triangles in perspective, in particular when the centre lies on the axis.

By the Artin–Zorn theorem every finite alternative division ring is a field, so every finite Moufang plane is Desarguesian.

Non-example: the Hall plane of order 99 is a finite non-Desarguesian plane. So by the previous sentence it is not a Moufang plane: the little Desargues property fails in it for some line ℓ\ell and point OO.

Reading path

To understand this page from the ground up, read these 3 pages in order. The foundations come first.

Show the path as a graph
Each page uses the pages above it. The colour of the bar on a box shows how strong the evidence is, and the box also gives it as text. The list has the same pages.
  • no evidence level
  1. Collinear points concept
  2. Projective plane concept
  3. Moufang plane definition

Further reading

  1. Web https://en.wikipedia.org/wiki/Moufang_plane
About this page
TagP5HCD49REV
URIm2://P5HCD49REV
Kinddefinition
WritingW0 stub
MSC51A35, 17D05
DefinesMoufang plane
UsesProjective plane, Collinear points

Markdown · JSON · Edit on GitHub

What this page needs for W1

  • a source with a locator (0 of 1)

The writing levels say how complete the text of a page is. They do not measure its truth.

What links here

No page links to this page yet.