Dear Dima, Willem, Peter, all, > Formulae for this can be found in Serre's book "Linear representations > of finite groups." Springer GTM, Vol. 42. I think Peter was asking for decomposition over Q. Let us stress "over Q"`. I agree Willem in that it is hard in general. For finite groups it mght be ber somewhat easier than the general case, but I am not aware of any serious result supportimg this. Gabor
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