Paper 2, Section II, 19F

Representation Theory | Part II, 2009

(i) Let GG be a finite group. Show that

(1) If χ\chi is an irreducible character of GG then so is its conjugate χˉ\bar{\chi}.

(2) The product of any two characters of GG is again a character of GG.

(3) If χ\chi and ψ\psi are irreducible characters of GG then

χψ,1G={1, if χ=ψˉ,0, if χψˉ.\left\langle\chi \psi, 1_{G}\right\rangle= \begin{cases}1, & \text { if } \chi=\bar{\psi}, \\ 0, & \text { if } \chi \neq \bar{\psi} .\end{cases}

(ii) If χ\chi is a character of the finite group GG, define χS\chi_{S} and χA\chi_{A}. For gGg \in G prove that

χS(g)=12(χ2(g)+χ(g2)) and χA(g)=12(χ2(g)χ(g2))\chi_{S}(g)=\frac{1}{2}\left(\chi^{2}(g)+\chi\left(g^{2}\right)\right) \quad \text { and } \chi_{A}(g)=\frac{1}{2}\left(\chi^{2}(g)-\chi\left(g^{2}\right)\right)

(iii) A certain group of order 24 has precisely seven conjugacy classes with representatives g1,,g7g_{1}, \ldots, g_{7}; further, GG has a character χ\chi with values as follows:

gig1g2g3g4g5g6g7CG(gi)242446666χ220ω2ωωω2\begin{array}{cccccccc} g_{i} & g_{1} & g_{2} & g_{3} & g_{4} & g_{5} & g_{6} & g_{7} \\ \left|C_{G}\left(g_{i}\right)\right| & 24 & 24 & 4 & 6 & 6 & 6 & 6 \\ \chi & 2 & -2 & 0 & -\omega^{2} & -\omega & \omega & \omega^{2} \end{array}

where ω=e2πi/3\omega=e^{2 \pi i / 3}.

It is given that g12,g22,g32,g42,g52,g62,g72g_{1}^{2}, g_{2}^{2}, g_{3}^{2}, g_{4}^{2}, g_{5}^{2}, g_{6}^{2}, g_{7}^{2} are conjugate to g1,g1,g2,g5,g4,g4,g5g_{1}, g_{1}, g_{2}, g_{5}, g_{4}, g_{4}, g_{5} respectively.

Determine χS\chi_{S} and χA\chi_{A}, and show that both are irreducible.

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