The Ramsey-ordering conjecture for permutation group orbits

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Let GG be a permutation group on Ω\Omega with no finite orbits. Assume that the orbits on nn-sets can be Ramsey ordered for every positive integer nn. For positive integers mm and nn, let cαc_\alpha, with α∈A\alpha\in\mathcal{A}, be a Ramsey ordering of the orbits on (m+n)(m+n)-sets with N≥2(m+n)N\ge 2(m+n), and let Xα⊆ΩX_\alpha\subseteq\Omega be corresponding sets. Suppose this ordering induces Ramsey orderings dβd_\beta, with β∈B\beta\in\mathcal{B}, and eγe_\gamma, with γ∈C\gamma\in\mathcal{C}, on the mm-set and nn-set orbits, respectively. Define β∨γ\beta\vee\gamma as the least index of an (m+n)(m+n)-set orbit containing a dβ∪eγd_\beta\cup e_\gamma decomposition.

Ramsey-ordering conjecture. There exists such a Ramsey ordering for which, for all β,β′∈B\beta,\beta'\in\mathcal{B} and γ,γ′∈C\gamma,\gamma'\in\mathcal{C},

β∨γ<β′∨γif β<β′andβ∨γ<β∨γ′if γ<γ′.\beta\vee\gamma<\beta'\vee\gamma\quad\text{if }\beta<\beta'\quad\text{and}\quad\beta\vee\gamma<\beta\vee\gamma'\quad\text{if }\gamma<\gamma'.

This conjecture seeks a compatible ordering of orbit decompositions that is strictly increasing in each component. The paper presents it as the main conjecture in the Ramsey-ordering section; no general resolution is given in the supplied text.

References

Primary source

Julian D. Gilbey, “Permutation group algebras”, arXiv:math/0112173 (2002).

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