Intransitive Cartesian decompositions preserved by innately transitive permutation groups

Authors:
Robert W. Baddeley, Cheryl E. Praeger and Csaba Schneider

Journal:
Trans. Amer. Math. Soc. **360** (2008), 743-764

MSC (2000):
Primary 20B05, 20B15, 20B25, 20B35

Published electronically:
August 28, 2007

MathSciNet review:
2346470

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Abstract: A permutation group is innately transitive if it has a transitive minimal normal subgroup, which is referred to as a plinth. We study the class of finite, innately transitive permutation groups that can be embedded into wreath products in product action. This investigation is carried out by observing that such a wreath product preserves a natural Cartesian decomposition of the underlying set. Previously we classified the possible embeddings in the case where the innately transitive group projects onto a transitive subgroup of the top group. In this article we prove that the transitivity assumption we made in the previous paper was not too restrictive. Indeed, the image of the projection into the top group can only be intransitive when the finite simple group that is involved in the plinth comes from a small list. Even then, the innately transitive group can have at most three orbits on an invariant Cartesian decomposition. A consequence of this result is that if is an innately transitive subgroup of a wreath product in product action, then the natural projection of into the top group has at most two orbits.

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Additional Information

**Robert W. Baddeley**

Affiliation:
32 Arbury Road, Cambridge CB4 2JE, United Kingdom

Email:
robert.baddeley@ntworld.com

**Cheryl E. Praeger**

Affiliation:
Department of Mathematics and Statistics, The University of Western Australia, 35 Stirling Highway, 6009 Crawley, Western Australia

Email:
praeger@maths.uwa.edu.au

**Csaba Schneider**

Affiliation:
Informatics Research Laboratory, Computer and Automation Research Institute, 1518 Budapest, Pf. 63, Hungary

Email:
csaba.schneider@sztaki.hu

DOI:
http://dx.doi.org/10.1090/S0002-9947-07-04223-7

Keywords:
Innately transitive groups,
plinth,
characteristically simple groups,
Cartesian decompositions,
Cartesian systems

Received by editor(s):
June 29, 2004

Received by editor(s) in revised form:
October 10, 2005

Published electronically:
August 28, 2007

Article copyright:
© Copyright 2007
American Mathematical Society