### Abstract

If V is a (possibly infinite) set, G a permutation group on V, ν ∈ V, and Ω is an orbit of the stabiliser G _{ν}, let G ^{Ω} _{ν} denote the permutation group induced by the action of G _{ν} on Ω, and let N be the normaliser of G in Sym(V). In this article, we discuss a relationship between the structures of G _{ν} and G ^{Ω} _{ν}. If G is primitive and G _{ν} is finite, then by a theorem of Betten et al. (J Group Theory 6:415-420, 2003) we can conclude that every composition factor of the group G _{ν} is also a composition factor of the group G ^{Ω(ν)} _{ν}. In this paper we generalize this result to possibly imprimitive permutation groups G with infinite vertex-stabilisers, subject to certain restrictions that can be expressed in terms of the natural permutation topology on Sym(V). In particular, we show the following: If Ω = u ^{Gν} is a suborbit of a transitive closed subgroup G of Sym(V) with a normalizing overgroup N ≤ N _{Sym(V)}(G) such that the N-orbital {(ν ^{g}, u ^{g}) {pipe} u ∈ Ω, g ∈ N} is locally finite and strongly connected (when viewed as a digraph on V), then every closed simple section of G _{ν} is also a section of G ^{Ω} _{ν}. To demonstrate that the topological assumptions on G and the simple sections of G _{ν} cannot be omitted in this statement, we give an example of a group G acting arc-transitively on an infinite cubic tree, such that the vertex-stabiliser G _{ν} is isomorphic to the modular group PSL(2, ℤ)≅ C _{2}*C _{3}, which is known to have infinitely many finite simple groups among its sections.

Original language | English (US) |
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Pages (from-to) | 497-504 |

Number of pages | 8 |

Journal | Monatshefte fur Mathematik |

Volume | 166 |

Issue number | 3-4 |

DOIs | |

State | Published - Jun 1 2012 |

### Keywords

- Locally-compact group
- Permutation group
- Point-stabiliser

### ASJC Scopus subject areas

- Mathematics(all)

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## Cite this

*Monatshefte fur Mathematik*,

*166*(3-4), 497-504. https://doi.org/10.1007/s00605-010-0282-0