Permutations on the Random Permutation

  • Julie Linman
  • Michael Pinsker
Keywords: Random permutation, Reduct, Ramsey theory, Canonical function


The random permutation is the Fraïssé limit of the class of finite structures with two linear orders. Answering a problem stated by Peter Cameron in 2002, we use a recent Ramsey-theoretic technique to show that there exist precisely 39 closed supergroups of the automorphism group of the random permutation, and thereby expose all symmetries of this structure. Equivalently, we classify all structures which have a first-order definition in the random permutation.

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