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    • CommentRowNumber1.
    • CommentAuthorMike Shulman
    • CommentTimeMay 6th 2015

    I made projective plane less stubby.

    • CommentRowNumber2.
    • CommentAuthorZhen Lin
    • CommentTimeMay 6th 2015

    A colleague of mine worked on a synthetic theory of projective planes over local rings for his thesis. If I recall correctly, he needed to add a variant of Pappus’s theorem as an axiom.

    • CommentRowNumber3.
    • CommentAuthorMike Shulman
    • CommentTimeMay 6th 2015

    Is there anything available about it?

    • CommentRowNumber4.
    • CommentAuthorDavidRoberts
    • CommentTimeMay 7th 2015

    Wasn’t that presented at CT2014?

    • CommentRowNumber5.
    • CommentAuthorDavidRoberts
    • CommentTimeMay 7th 2015
    • (edited May 7th 2015)

    There’s also the result of Ruth Moufang about coordinatising projective planes satisfying (edit: little) Desargues with alternative division rings. She discovered OP 2OP^2 this way, I gather.

    • CommentRowNumber6.
    • CommentAuthorMike Shulman
    • CommentTimeMay 8th 2015

    One thing I don’t yet understand: a projective plane without any Desargues’ theorem can be constructed/coordinatized using a “ternary ring” (which isn’t actually a ring at all, but an algebraic gadget whose basic operation is ternary, thought of as like (a,b,c)ab+c(a,b,c) \mapsto a b + c). Is the construction of OP 2O P^2 a special case of that?

    • CommentRowNumber7.
    • CommentAuthorDavidRoberts
    • CommentTimeMay 8th 2015

    OP 2OP^2 is a Moufang plane, so little Desargues’ holds (Theorem 3.8 here). This is strictly weaker than the usual Desargues’ theorem. Thanks for pointing out planar ternary rings. My officemate as a PhD student was a finite geometer, so I learned a bit by osmosis about non-Desarguesian planes, but not about these.

    • CommentRowNumber8.
    • CommentAuthorDavid_Corfield
    • CommentTimeMay 8th 2015

    Did John Baez’s account cover this? Here he is on Octonionic Projective Geometry.

    • CommentRowNumber9.
    • CommentAuthorDavidRoberts
    • CommentTimeMay 8th 2015
    • (edited May 8th 2015)

    JB says

    in 1933 Ruth Moufang constructed a remarkable example of a non-Desarguesian projective plane using the octonions

    Here’s Moufang’s paper he cites:

    Ruth Moufang, Alternativkörper und der Satz vom vollständigen Vierseit, Abh. Math. Sem. Hamburg 9 (1933), 207-222. doi:10.1007/BF02940648

    • CommentRowNumber10.
    • CommentAuthorMike Shulman
    • CommentTimeMay 8th 2015

    Right, I know all that. Since OP 2O P^2 is non-Desarguesian, we can construct from it a ternary ring that coordinatizes it — but my question is, can the original construction of OP 2O P^2 from OO be obtained by way of regarding OO as a ternary ring? I presume so, but in that case it surprises me that hardly anyone seems to mention this when discussing OP 2O P^2, so I end up with doubt.

    • CommentRowNumber11.
    • CommentAuthorDavidRoberts
    • CommentTimeMay 8th 2015
    • (edited May 8th 2015)

    From the wikipedia article on Moufang planes I linked to, we find characterisation of them:

    Some ternary ring of the plane is an alternative division ring.

    P is isomorphic to the projective plane over an alternative division ring.

    Also, in a Moufang plane:

    Any two ternary rings of the plane are isomorphic.

    Since O, as a ternary ring, is an alternating ring, any other ternary ring coordinatising OP 2OP^2 is isomorphic to O.

    When you say the original construction, I guess one would have to dig up either a translation of Moufang’s construction, or read it in German. My guess is that the ternary ring version reproduces Moufang’s result in the alternating ring case in general, not just for O.

    • CommentRowNumber12.
    • CommentAuthorMike Shulman
    • CommentTimeMay 8th 2015

    Oh good.

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