Permutation representations and rational irreducibility

Abstract

The natural character π of a finite transitive permutation group G has the form 1G + θ where θ is a character which affords a rational representation of G. We call G a QI-group if this representation is irreducible over ℚ. Every 2-transitive group is a QI-group, but the latter class of groups is larger. It is shown that every QI-group is ¾-transitive and primitive, and that it is either almost simple or of affine type. QI-groups of affine type are completely determined relative to the 2-transitive affine groups, and partial information is obtained about the socles of simply transitive almost simple QI-groups. The only known simply transitive almost simple QI-groups are of degree 2k−1(2k − 1) with 2k − 1 prime and socle isomorphic to PSL(2, 2k).