The Unapologetic Mathematician

Mathematics for the interested outsider

Characters of Induced Representations

We know how to restrict and induce representations. Now we want to see what this looks like on the level of characters.

For restricted representations, this is easy. Let X be a matrix representation of a group G, and let H\subseteq G be a subgroup. Then X\!\!\downarrow^G_H(h)=X(h) for any h\in H. We just consider an element of H as an element in G and construct the matrix as usual. Therefore we can see that

\displaystyle\begin{aligned}\chi\!\!\downarrow^G_H(h)&=\mathrm{Tr}\left(X\!\!\downarrow^G_H(h)\right)\\&=\mathrm{Tr}\left(X(h)\right)\\&=\chi(h)\end{aligned}

That is, we get the restricted character by restricting the original character.

As for the induced character, we use the matrix of the induced representation that we calculated last time. If X is a matrix representation of a group H, which is a subgroup H\subseteq G, then we pick a transversal of H in G. Using our formula for the induced matrix, we find

\displaystyle\begin{aligned}\chi\!\!\uparrow_H^G(g)&=\mathrm{Tr}\left(X\!\!\uparrow_H^G(g)\right)\\&=\mathrm{Tr}\left(\begin{array}{cccc}X(t_1^{-1}gt_1)&X(t_1^{-1}gt_2)&\cdots&X(t_1^{-1}gt_n)\\X(t_2^{-1}gt_1)&X(t_2^{-1}gt_2)&\cdots&X(t_2^{-1}gt_n)\\\vdots&\vdots&\ddots&\vdots\\X(t_n^{-1}gt_1)&X(t_n^{-1}gt_2)&\cdots&X(t_n^{-1}gt_n)\end{array}\right)\\&=\sum\limits_{i=1}^n\mathrm{Tr}\left(X(t_i^{-1}gt_i)\right)\\&=\sum\limits_{i=1}^n\chi(t_i^{-1}gt_i)\end{aligned}

Where we define \chi(g)=0 if g\notin H. Now, since \chi is a class function on H, conjugation by any element h\in H leaves it the same. That is,

\displaystyle\chi(h^{-1}gh)=\chi(g)

for all g\in G and h\in H. So let’s do exactly this for each element of H, add all the results together, and then divide by the number of elements of H. That is, we write the above function out in \lvert H\rvert different ways, add them all together, and divide by \lvert H\rvert to get exactly what we started with:

\displaystyle\begin{aligned}\chi\!\!\uparrow_H^G(g)&=\frac{1}{\lvert H\rvert}\sum\limits_{h\in H}\sum\limits_{i=1}^n\chi(h^{-1}t_i^{-1}gt_ih)\\&=\frac{1}{\lvert H\rvert}\sum\limits_{h\in H}\sum\limits_{i=1}^n\chi\left((t_ih)^{-1}g(t_ih)\right)\end{aligned}

But now as t_i varies over the transversal, and as h varies over H, their product t_ih varies exactly once over G. That is, every x\in G can be written in exactly one way in the form t_ih for some transversal element t_i and subgroup element h. Thus we find:

\displaystyle\chi\!\!\uparrow_H^G(g)=\frac{1}{\lvert H\rvert}\sum\limits_{x\in G}\chi(x^{-1}gx)

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November 29, 2010 - Posted by | Algebra, Group theory, Representation Theory

1 Comment »

  1. [...] we can prove the Frobenius’ reciprocity formula, which relates induced characters to restricted [...]

    Pingback by (Fake) Frobenius Reciprocity « The Unapologetic Mathematician | November 30, 2010 | Reply


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