## Cycle action on treelike structures.

##### J. Algebr. Comb. 22(1), 105-114 (2005)
DOI: 10.1007/s10801-005-7395-x

### Summary

Summary: The purpose of this paper is to study the action on cycles of several known classes of oligomorphic groups, that is, infinite permutation groups of countable degree having only finitely many orbits on $k$-sets for each $k$. The groups studied here are all related to trees and treelike relational structures. The sequence whose $k$-th term is the number of orbits in the action on $k$-cycles is called Parker sequence. It turns out that, if we are dealing with the automorphism group of a suitable relational structure, this sequence counts also the finite substructures admitting a cyclic automorphism; in calculating these sequences for various groups, we shall thus describe and enumerate such substructures. Di più dirò: ch'a gli alberi dà vita [I shall say more: the trees are given life spirito uman che sente e che ragiona. by a human spirit that perceives and reasons.

### Keywords/Phrases

keywords oligomorphic permutation groups, action on cycles, circulant relational structures, trees