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Geometric interpretations of the ‘natural’ generators of the Mathieu groups

Published online by Cambridge University Press:  24 October 2008

R. T. Curtis
School of Mathematics and Statistics, University of Birmingham, Birmingham B12 2TT


In [1] we used the alternating group A5 to produce a set of five permutations of order three acting on 12 letters; this set was normalized by A5 and generated the Mathieu group M12. We analogously used the linear group PSL2(7) to produce a set of seven involutions acting on 24 letters which was normalized by our PSL2(7) and generated M24. In this paper we describe the combinatorial and geometric interpretations of our generators, and aim to justify our use of the word ‘natural’. The generators for M12 are defined both as permutations of twelve 5-cycles in a conjugacy class of A5, and as permutations of the faces of a dodecahedron. The outer automorphism of M12 emerges as a bonus. The generators of M24 are defined analogously both as permutations of twenty-four 7-cycles in the action of PSL2(7) on seven letters, and as permutations of the 24 faces of a certain cubic graph drawn on a surface of genus three. In addition we mention the connection between the latter construction and the binary Golay code.

Research Article
Copyright © Cambridge Philosophical Society 1990

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[1]Curtis, R. T.. Natural constructions of the Mathieu groups. Math. Proc. Cambridge Philos. Soc. 106 (1989), 423430.Google Scholar
[2]Curtis, R. T.. M 24 and related topics. Ph.D. thesis, University of Cambridge (1972).Google Scholar