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Proceedings of the London Mathematical Society Advance Access published online on August 25, 2009
Proceedings of the London Mathematical Society, doi:10.1112/plms/pdp033
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© 2009 London Mathematical Society

Wilson's map operations on regular dessins and cyclotomic fields of definition

G. A. Jones

School of Mathematics
University of Southampton
Southampton
SO17 1BJ
United Kingdom

M. Streit

Usinger Str. 56
D–61440 Oberursel
Germany
Manfred.Streit@bahn.de

J. Wolfart

Mathematisches Seminar der Goethe Universität
Postfach 111932
D–60054 Frankfurt a.M.
Germany
wolfart@math.uni-frankfurt.de

Received 22 September 2008. Revision received 9 June 2009.

Dessins d’enfants can be seen as bipartite graphs embedded in compact orientable surfaces. According to Grothendieck and others, a dessin uniquely determines a complex structure on the surface, even an algebraic structure as a projective algebraic curve defined over a number field. Combinatorial properties of the dessin should therefore determine the equations and also structural properties of the curve, such as the field of moduli or the field of definition. However, apart from a few series of examples, very few general results concerning such correspondences are known. As a step in this direction, we present a graph theoretic characterisation of certain quasiplatonic curves defined over cyclotomic fields, based on Wilson's operations on maps: these leave invariant the graph but change the cyclic ordering of edges around the vertices, therefore they change the embeddings, and hence the dessins, and hence the conformal and algebraic structure of the underlying curves. Under suitable assumptions, satisfied by many series of regular dessins, these changes coincide with the effect of Galois conjugation. This coincidence allows one to draw conclusions about Galois orbits and fields of definition of dessins. The possibilities afforded by these techniques, and their limitations, are illustrated by a new look at some known examples and a study of dessins based on the regular embeddings of complete graphs.

2000 Mathematics Subject Classification 14H45 (primary), 14H25, 14H55, 05C10, 05C25, 30F10 (secondary).

During the preparation of the article the third author was supported by the Deutsche Forschungsgemeinschaft with the projects Wo 199/3 and Wo 199/4.


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