Splitting fields of representations of generalized symmetric groups, 8

Posted on 2016/01/08 by wdjoyner

In this post, we give an example.

Let $G=C_3^8\, >\!\!\lhd \, S_8$ and let

$\pi = \theta_{\mu,\rho}=Ind_{G_\mu}^G(\mu\cdot \tilde{\rho}),$

where $\mu$ is a character of $C_3^8$ and $\rho$ is an irreducible representation of its stabilizer in $S_8$ , $(S_8)_\mu$ .

The real representations $\pi$ of $G$ are the ones for which

$\mu$ is represented by a character of the form
$(1,1,1,1,1,1,\omega,\omega^2) \ {\rm or}\ (1,1,...,1),$

and $\rho$ anything, or
$\mu$ is represented by a character of the form
$(1,1,1,1,\omega,\omega,\omega^2,\omega^2), \rho_1=(\pi_1,\pi_2,\pi_2)\in (S_4)^*\times (S_2)^*\times (S_2)^*,$

or
$\mu$ is represented by a character of the form
$(\omega,\omega,\omega,\omega,\omega^2,\omega^2,\omega^2,\omega^2), \rho_1=(\pi_2,\pi_2)\in (S_4)^*\times (S_4)^*,$

or
$\mu$ is represented by a character of the form
$(1,1,\omega,\omega,\omega,\omega^2,\omega^2,\omega^2), \rho_1=(\pi_1,\pi_2,\pi_2)\in (S_2)^*\times (S_3)^*\times (S_3)^*.$

The complex representations of $G$ are: the representations
whose characters have at least one complex value. Such representations $\pi = \theta_{\mu,\rho}$ are characterized by the fact that $(\mu,\rho)$ is inequivalent to $(\overline{\mu},\rho)$ under the obvious $S_8$-equivalence relation (which can be determined from the equivalence relation for representations in $G^*$ ).

The complex representations of $G$ are the remaining representations not included in the above list.

There are no quaternionic representations of $G$ .

The claims above follow from the fact that a representation
$\theta_{\rho,\mu}$ is complex if and only if $\mu$ is not self-dual.

Splitting fields of representations of generalized symmetric groups, 7

Posted on 2016/01/07 by wdjoyner

In this post, we discover which representations of the generalized symmetric group $G = S_n\ wr\ C_\ell = C_\ell^n\, >\!\!\lhd \, S_n$ can be realized over a given abelian extension of ${\mathbb{Q}}$ .

Let $\theta_{\mu,\rho}\in G^*$ be the representation defined previously, where $\rho\in ((S_n)_\mu)^*$ .

Let $K\subset {\mathbb{Q}}(\zeta_\ell)$ be a subfield, where $\zeta_\ell$ is a primitive $\ell^{th}$ root of unity. Assume $K$ contains the field generated by the values of the character of $\theta_{\mu,\rho}$ . Assume $K/{\mathbb{Q}}$ is Galois and let $\Gamma_K=Gal({\mathbb{Q}}(\zeta_\ell)/K)$ . Note if we regard $C_\ell$ as a subset of ${\mathbb{Q}}(\zeta_\ell)$ then there is an induced action of $\Gamma_K$ on $C_\ell$ ,

$\sigma:\mu \longmapsto \mu^\sigma, \ \ \ \ \ \ \ \ \ \mu\in (C_\ell)^*,\ \ \sigma\in \Gamma_K,$

where $\mu^\sigma(z)=\mu(\sigma^{-1}(z))$ , $z\in C_\ell$ . This action extends to an action on $(C_\ell^n)^*=(C_\ell^*)^n$ .

Key Lemma:
In the notation above, $\theta_{\mu,\rho}\cong\theta_{\mu,\rho}^\sigma$ if and only if $\mu$ is equivalent to $\mu^\sigma$ under the action of $S_n$ on $(C_\ell^n)^*$ .

Let

$n_\mu(\chi)=|\{i\ |\ 1\leq i\leq n,\ \mu_i=\chi\}|,$

where $\mu=(\mu_1,...,\mu_n)\in (C_\ell^n)^*$ and $\chi\in C_\ell^*$ .

Theorem: The character of $\theta_{\mu,\rho}\in G^*$ has values in $K$ if and only if $n_\mu(\chi)=n_\mu(\chi^\sigma)$ ,
for all $\sigma\in \Gamma_K$ and all $\chi\in C_\ell^*$ .

This theorem is proven in this paper.

We now determine the splitting field of any irreducible character of a generalized symmetric group.

Theorem: Let $\chi=tr(\theta_{\rho,\mu})$ be an irreducible character of $G=S_n\ wr\ C_\ell$ . We have

$Gal({\mathbb{Q}}(\zeta_\ell)/{\mathbb{Q}}(\chi))= Stab_\Gamma(\chi).$

This theorem is also proven in this paper.

In the next post we shall give an example.

Splitting fields of representations of generalized symmetric groups, 6

Posted on 2016/01/06 by wdjoyner

This post shall list some properties of the Schur index $m_F(G)$ in the case where $G = S_n\ wr\ C_\ell$ is a generalized symmetric group and $F$ is either the reals or rationals.

Let $\eta_k(z)=z^k$ , for $z\in C_\ell$ , $1\leq k\leq \ell$ .

Theorem: Let $G = S_n\ wr\ C_\ell$ . Let $\mu=(\eta_{e_1},...,\eta_{e_n})\in (C_\ell^n)^*$ , for some $e_j\in \{0,...,\ell-1\}$ , and let $\rho\in (S_n)_\mu^*$ . Let
$\chi$ denotes the character of $\theta_{\mu,\rho}$ .

Suppose that one of the following conditions holds:
1. $4|\ell$ and $\overline{e_1+...+e_n}$ divides $\overline{\ell/4}$ in ${\mathbb{Z}}/\ell {\mathbb{Z}}$ , or
2. $(e_1+...+e_n,\ell)=1$ ,
Then $m_{\Bbb{Q}}(\chi)=1$ .
Suppose that one of the following conditions holds:
1. $(n,\ell)=1$ , $4|\ell$ , and $(e_1+...+e_n)x\equiv \ell /4\ ({\rm mod}\ \ell)$ is not solvable, or
2. $(n,\ell)=1$ and $(e_1+...+e_n,\ell)>1$ .
Then $m_{\mathbb{Q}}(\chi\eta_1)=1$ .

This theorem is proven in this paper. Benard has shown that $m_{\mathbb{Q}}(\chi)=1$ , for all $\chi$ as in the above theorem.

Since the Schur index over ${\mathbb{Q}}$ of any irreducible character $\chi$ of a generalized symmetric group $G$ is equal to $1$ , each such character is associated to a representation $\pi$ all of whose matrix coefficients belong to the splitting field ${\mathbb{Q}}(\chi)$ .

What is the splitting field ${\mathbb{Q}}(\chi)$ , for $\chi\in G^*$ ?

This will be addressed in the next post.

Splitting fields of representations of generalized symmetric groups, 5

Posted on 2016/01/03 by wdjoyner

It is a result of Benard (Schur indices and splitting fields of the unitary reflection groups, J. Algebra, 1976) that the Schur index over ${\mathbb{Q}}$ of any irreducible character of a generalized symmetric group is equal to $1$ . This post recalls, for the sake of comparison with the literature, other results known about the Schur index in this case.

Suppose that $G$ is a finite group and $\pi \in G^*$ is an irreducible representation of $G$ , $\pi :G\rightarrow Aut(V)$ , for some complex vector space $V$ . We say that $\pi$ may be realized over a subfield $F\subset {\mathbb{C}}$ if there is an $F$ -vector space $V_0$ and an action of $G$ on $V_0$ such that $V$ and ${\mathbb{C}}\otimes V_0$ are equivalent representations of $G$ , where $G$ acts on ${\mathbb{C}}\otimes V_0$ by “extending scalars” in $V_0$ from $F$ to ${\mathbb{C}}$ . Such a representation is called an $F$ -representation. In other words, $\pi$ is an $F$ -representation provided it is equivalent to a representation which can be written down explicitly using matrices with entries in $F$ .

Suppose that the character $\chi$ of $\pi$ has the property that

$\chi(g)\in F, \ \ \ \ \ \ \forall g\in G,$

for some subfield $F\subset {\mathbb{C}}$ independent of $g$ . It is unfortunately true that, in general, $\pi$ is not necessarily an $F$ -representation. However, what is remarkable is that, for some $m\geq 1$ , there are $m$ representations, $\pi_1,...,\pi_m$ , all equivalent to $\pi$ , such that $\pi_1\oplus ...\oplus \pi_m$ is an $F$ -representation. The precise theorem is the following remarkable fact.

Theorem: (Schur) Let $\chi$ be an irreducible character and let $F$ be any field containing the values of $\chi$ . There is an integer $m \geq 1$ such that $m\chi$ is the character of an $F$ -representation.

The smallest $m\geq 1$ in the above theorem is called the Schur index and denoted $m_F(\chi)$ .

Next, we introduce some notation:

let ${\mathbb{R}}(\pi) = {\mathbb{R}}(\chi)$ denote the extension field of ${\mathbb{R}}$ obtained by adjoining all the values of $\chi(g)$ \ ($g\in G$), where $\chi$ is the character of $\pi$ ,
let $\nu(\pi) = \nu(\chi)$ denote the Frobenius-Schur indicator of $\pi$ (so $\nu(\pi)= {1\over |G|}\sum_{g\in G} \chi(g^2)$ ),
let $m_{\mathbb{R}}(\pi) = m_{\mathbb{R}}(\chi)$ denote the Schur multiplier of $\pi$ (by definition, the smallest integer $m\geq 1$ such that $m\chi$ can be realized over ${\mathbb{R}}$ (this integer exists, by the above-mentioned theorem of Schur).

The following result shows how the Schur index behaves under induction (see Proposition 14.1.8 in G. Karpilovsky,
Group representations, vol. 3, 1994).

Proposition: Let $\chi$ be an irreducible character of $G$ and let $\psi$ denote an irreducible character of a subgroup $H$ of $G$ . If $= 1$ then $m_{\Bbb{R}}(\chi)$ divides $m_{\Bbb{R}}(\psi)$ .

A future post shall list some properties of the Schur index in the case where $G$ is a generalized symmetric group and $F$ is either the reals or rationals.

Splitting fields of representations of generalized symmetric groups, 4

Posted on 2016/01/01 by wdjoyner

This post if an aside on cyclotomic fields and Tchebysheff polynomials. Though it seems certain this material is known, I know of no reference.

Let $n$ denote a positive integer divisible by $4$ , let $r=\cos(2\pi/n)$ , $s=\sin(2\pi/n)$ , and let $d=n/4$ . If

$T_1(x)=x,\ \ T_2(x)=2x^2-1,\ \ T_3(x)=4x^3-3x,\ \ T_4(x)=8x^4-8x^2+1,\ \ ...,$

denote the Tchebysheff polynomials (of the 1st kind), defined by

$\cos(n\theta)=T_n(\cos(\theta)),$

then $T_d(r)=0.$

Let $\zeta_n=exp(2\pi i/n)$ and let $F_n={\mathbb{Q}}(\zeta_n)$ denote the cyclotomic field of degree $\phi(n)$ over ${\mathbb{Q}}$ . If $\sigma_j\in Gal(F_n/{\Bbb{Q}})$ is defined by $\sigma_j(\zeta_n)=\zeta_n^j$ then

$Gal(F_n/{\Bbb{Q}})\cong ({\Bbb{Z}}/n{\Bbb{Z}})^\times,$

where $\sigma_j\longmapsto j$ .

Lemma: Assume $n$ is divisible by $4$ .

${\mathbb{Q}}(r)$ is the maximal real subfield of $F_n$ , Galois over ${\mathbb{Q}}$ with
$Gal(F_n/{\Bbb{Q}}(r))=\{1,\tau\},$

where $\tau$ denotes complex conjugation. Under the canonical isomorphism

$Gal(F_n/{\Bbb{Q}})\cong ({\Bbb{Z}}/n{\Bbb{Z}})^\times,$

we have

$Gal({\Bbb{Q}}(r)/{\Bbb{Q}})\cong ({\Bbb{Z}}/n{\Bbb{Z}})^\times/\{\pm 1\}.$
If $n$ is divisible by $8$ then $r$ and $s$ are conjugate roots of $T_d$ . In particular, $s\in {\mathbb{Q}}(r)$ and $T_d(s)=0$ .
We have $\sigma_j(r)=T_j(r)$ .
If $n\geq 4$ is a power of $2$ then $T_d$ is the minimal polynomial of ${\mathbb{Q}}(r)$ . Furthermore, in this case
$\cos(\pi/4)=\sqrt{2}/2,\ \ \cos(\pi/8)=\sqrt{2+\sqrt{2}}/2,\ \ \cos(\pi/16)=\sqrt{2+\sqrt{2+\sqrt{2}}}/2,\ \ ... .$