← All papers
First page of EPPA for Reducts of the Random Graph

EPPA for Reducts of the Random Graph

Sofia Brenner, Jan Hubička, Matěj Konečný

math.CO Oct 29, 2026 · v1 cs.DM math.LO
The main theorem and Proposition 5.2 were formalised in Lean 4, largely autonomously by AI, as a check on the proofs; a public repository is provided.
We prove that finite two-graphs have coherent EPPA and that neither graphs modulo global complementation nor two-graphs modulo global complementation have EPPA. This completes the classification of EPPA and coherent EPPA for the reducts of the random graph, answering a question and confirming a conjecture of Evans, Hubička, Konečný and Nešetřil.

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 structuresEPPAcoherent EPPA
G (graphs)yesyes
S (sets)yesyes
T (two-graphs)yesyes
G±nono
T±nono
EPPA classification for reducts of the random graph