Structures with Small Orbit Growth

Abstract

Let $K_{exp+}$ be the class of all structures $A$ such that the automorphism group of $A$ has at most $c n^{d n}$ orbits in its componentwise action on the set of $n$-tuples with pairwise distinct entries, for some constants $c,d$ with $d < 1$. We show that $K_{exp+}$ is precisely the class of finite covers of first-order reducts of unary structures, and also that $K_{exp+}$ is precisely the class of first-order reducts of finite covers of unary structures. It follows that the class of first-order reducts of finite covers of unary structures is closed under taking model companions and model-complete cores, which is an important property when studying the constraint satisfaction problem for structures from $K_{exp+}$. We also show that Thomas' conjecture holds for $K_{exp+}$: all structures in $K_{exp+}$ have finitely many first-order reducts up to first-order interdefinability.