Talk:Faithful representation

I have corrected the "caveat". There is a more obvious thing to say, namely that if g is represented by −I then the representation will not be a faithful module, since e + g is represented by 0. The example given has the virtue of independent interest, I think. Charles Matthews (talk) 10:27, 17 March 2010 (UTC)

• Thanks a lot for fixing the spam I wrote. I mistook tensor powers for direct powers and thought I had a proof that every faithful representation is a faithful module in the classical (non-modular) case. The only thing I don't understand is why you need Cayley's theorem; in my opinion, the permutation representation of $S_{n}$ is faithful because a linear map is uniquely determined by its action on a basis. But it's trivial one way or another. -Darij (talk) 15:53, 17 March 2010 (UTC)

The $V^{\otimes n}$ property

I'd be indebted if someone finds a reference for it. The result that if V is faithful, then every irrep occurs inside $V^{\otimes n}$ is very well-known (e. g., Fulton-Harris), but I've seen the stronger result that every irrep occurs inside $S^{n}V$ only as problem 3.26 in Etingof's Representation lectures. As for the converse (if every irrep occurs inside $V^{\otimes n}$, then V is faithful), it's easy to prove (in fact, $V^{\otimes n}$ is faithful, because otherwise a group element would act trivially on each irrep, and thus on the group ring; hence, V must also be faithful).

Also, it seems to me that neither the tensor product of two representations, nor symmetric powers of representations are ever defined on Wikipedia. (Except for the tensor product in the more general Hopf algebraic context.) -Darij (talk) 16:06, 17 March 2010 (UTC)