EPPA for Reducts of the Random Graph
Finish classifying the extension property for partial automorphisms (EPPA) and coherent EPPA for the five reducts of the random graph. The open cases were two-graphs, graphs modulo global complementation, and two-graphs modulo global complementation, answering a question of Evans, Hubička, Konečný and Nešetřil.
A six-vertex obstruction with two complementing partial automorphisms, combined with a fixed-point counting lemma for permutation groups, shows that graphs modulo complementation lack EPPA. Rooting two-graphs reduces the two-graph complementation case to the graph case. Coherent EPPA for two-graphs is proved with a modified valuation-function construction that quotients at the level of valuation functions and uses a large block-divisible index set. The main results were formalised in Lean 4, mostly autonomously by AI.
Finite two-graphs have coherent EPPA. Neither graphs modulo global complementation nor two-graphs modulo global complementation have EPPA. This completes the classification for the five reducts.
| finite structures | EPPA | coherent EPPA |
|---|---|---|
| G (graphs) | yes | yes |
| S (sets) | yes | yes |
| T (two-graphs) | yes | yes |
| G± | no | no |
| T± | no | no |
