Timeline for Can one axiomatize projective lines using the cross-ratio?
Current License: CC BY-SA 3.0
5 events
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| Apr 13, 2017 at 12:57 | history | edited | CommunityBot |
replaced http://mathoverflow.net/ with https://mathoverflow.net/
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| Nov 1, 2016 at 13:10 | comment | added | David Roberts♦ | There's work by Anders Kock on projective lines that might be worth looking at. I only remembered this recently but didn't get to see how his ideas relate to the stuff Mike put on the nLab. | |
| Nov 1, 2016 at 8:43 | answer | added | MvG | timeline score: 3 | |
| Nov 1, 2016 at 3:04 | comment | added | Andreas Blass | Something doesn't "feel" right here. The cross-ratio is invariant under the action of PGL$(2,k)$ on $kP^1$ by fractional linear transformations. So in $kP^1\times kP^1\times kP^1\times kP^1\to kP^1$, you'd have PGL$(2,k)$ acting by fractional linear transformations on the four factors of the domain but acting trivially on the codomain. Is that really what you want? Or what am I missing? | |
| Oct 31, 2016 at 23:54 | history | asked | John Baez | CC BY-SA 3.0 |