Let $G$ be a finite group, and $H$ be a subgroup of $G$.
If I am not mistaken, we have
$$Ind_H^G \,\,1_H=1_G+\sum_{\chi\neq 1} n_\chi \chi$$
where $\chi$ are irreducible representations.
My question is, what representations are these?
I guess is should be a proper subset (if $H$ is a proper subgroup) of the irreducible representations of $G$, but I'm not too sure of that either.
And if that's correct, how do I know which $\chi$ occur in the decomposition above.
Best Answer
One way to answer this is given by what is called Frobenius reciprocity (for simplicity, I am assuming that we are working over the complex numbers here).
This states that the constituents of the induced representation are precisely those which, when restricted to the subgroup, have the trivial representation as a constituent.
The above also works for any other irreducible representation of the subgroup, and even says what the multiplicities are (being the same as the multiplicity when one restricts).