Séminaire Lotharingien de Combinatoire, 80B.83 (2018), 12 pp.

Justine Falque and Nicolas M. Thiéry

The Orbit Algebra of an Oligomorphic Permutation Group with Polynomial Profile is Cohen-Macaulay

Abstract. Let G be a group of permutations of a denumerable set E. The profile of G is the function φG which counts, for each n, the (possibly infinite) number φG(n) of orbits of G acting on the n-subsets of E. Counting functions arising this way, and their associated generating series, form a rich yet apparently strongly constrained class. In particular, Cameron conjectured in the late seventies that, whenever φG(n) is bounded by a polynomial, it is asymptotically equivalent to a polynomial. In 1985, Macpherson further asked if the \textbf{orbit algebra} of G - a graded commutative algebra invented by Cameron and whose Hilbert function is φG - is finitely generated.

In this paper we announce a proof of a stronger statement: the orbit algebra is Cohen Macaulay; it follows that the generating series of the profile is a rational fraction whose denominator admits a combinatorial description and the numerator is non-negative.

The proof uses classical techniques from actions of permutation groups, commutative algebra, and invariant theory; it steps towards a classification of ages of permutation groups with profile bounded by a polynomial.

Received: November 14, 2017. Accepted: February 17, 2018. Final version: April 1, 2018.

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