1.II.19G

Representation Theory
Part II, 2008

For a complex representation VV of a finite group GG, define the action of GG on the dual representation VV^{*}. If α\alpha denotes the character of VV, compute the character β\beta of VV^{*}.

[Your formula should express β(g)\beta(g) just in terms of the character α\alpha.]

Using your formula, how can you tell from the character whether a given representation is self-dual, that is, isomorphic to the dual representation?

Let VV be an irreducible representation of GG. Show that the trivial representation occurs as a summand of VVV \otimes V with multiplicity either 0 or 1 . Show that it occurs once if and only if VV is self-dual.

For a self-dual irreducible representation VV, show that VV either has a nondegenerate GG-invariant symmetric bilinear form or a nondegenerate GG-invariant alternating bilinear form, but not both.

If VV is an irreducible self-dual representation of odd dimension nn, show that the corresponding homomorphism GGL(n,C)G \rightarrow G L(n, \mathbf{C}) is conjugate to a homomorphism into the orthogonal group O(n,C)O(n, \mathbf{C}). Here O(n,C)O(n, \mathbf{C}) means the subgroup of GL(n,C)G L(n, \mathbf{C}) that preserves a nondegenerate symmetric bilinear form on Cn\mathbf{C}^{n}.