Dear Dima, Peter, All, In order to decompose a rational module, one can compute its endomorphism algebra (i.e., all linear maps that comute with the action of G). In this algebra one can find the central idempotents. (That particular step is also implemented in GAP). Those will give one the components that are direct sums of isotypical modules. However, if the endomorphism algebra contains subalgebras that are isomorphic to full matrix algebras, then one would have to find complete sets of idempotents also in those algebras. That is an extremely hard problem in general.
I hope the above makes sense. All the best, Willem _______________________________________________ Forum mailing list [email protected] http://mail.gap-system.org/mailman/listinfo/forum
