# Representation Theory problems -->

• Apr 23rd 2009, 11:29 AM
TreyD0g
Representation Theory problems -->
i've got a few adv. linear algebra problems that i need to do, but i'm not real knowledgeable on representation theory.
here is the first and easiest problem of the bunch: problem #1

also, with that i need to find the trace and norm of 2 - 3i.

any help would be greatly appreciated.
• Apr 30th 2009, 12:40 PM
TreyD0g
here's another problem i'm working on: http://i475.photobucket.com/albums/r...de0flife/2.jpg
• Apr 30th 2009, 02:23 PM
NonCommAlg
Quote:

Originally Posted by TreyD0g
i've got a few adv. linear algebra problems that i need to do, but i'm not real knowledgeable on representation theory.
here is the first and easiest problem of the bunch: problem #1

also, with that i need to find the trace and norm of 2 - 3i.

any help would be greatly appreciated.

let $B=\{\alpha_1, \alpha_2 \}$ and $B'=\{1,i \},$ the standard basis of $\mathbb{C}$ over $\mathbb{R}.$ then $[T_{\alpha}]_B=P[T_{\alpha}]_{B'}P^{-1},$ for some $2 \times 2$ matrix $P.$ thus $N(\alpha)=\det [T_{\alpha}]_{B'}$ and $Tr(\alpha)=Tr [T_{\alpha}]_{B'}.$

now let $\alpha=x+yi, \ \beta=z+ti.$ then $T_{\alpha}(1)=\alpha=x+yi$ and $T_{\alpha}(i)=\alpha i = -y + xi,$ which gives us: $[T_{\alpha}]_{B'}=\begin{bmatrix}x & -y \\ y & x \end{bmatrix}.$ thus $N(\alpha)=x^2+y^2$ and $Tr(\alpha)=2x.$

similarly $N(\beta)=z^2+t^2$ and $Tr(\beta)=2z.$ now we have: $\alpha \beta = xz - yt + (xt + yz)i$ and hence: $N(\alpha \beta)=(xz - yt)^2 + (xt + yz)^2=(x^2 + y^2)(z^2 + t^2)=N(\alpha) N(\beta).$

this proves part (a) of your problem. also it's clear that $N(2 - 3i)=4+9=13$ and $Tr(2-3i)=4.$
• May 3rd 2009, 10:04 AM
TreyD0g
ok, this problem has given me the most trouble so far. i'm not really sure where to start:
http://i475.photobucket.com/albums/r...ber3ans010.jpg

and again, i still haven't been able to solve this problem: http://i475.photobucket.com/albums/r...de0flife/2.jpg
• May 3rd 2009, 01:39 PM
NonCommAlg
Quote:

Originally Posted by TreyD0g

ok, this problem has given me the most trouble so far. i'm not really sure where to start:
http://i475.photobucket.com/albums/r...ber3ans010.jpg

since $G$ is a finite group, the order of $a$ is finite, i.e. $a^m=1_G,$ for some integer $m.$ now if $\lambda$ is an eigenvalue of $\rho(a),$ then $\rho(a)x=\lambda x,$ for some non-zero vector $x.$

thus $x=\rho(1_G)x=\rho(a^m)x=\lambda^m x.$ hence $\lambda^m=1.$ so every eigenvalue $\lambda$ of $\rho(a)$ is a root of unity and thus $\frac{1}{\lambda}=\overline{\lambda}.$ now $\lambda$ is an eigenvalue of $\rho(a^{-1})$ if and only if

$\rho(a^{-1})x=\lambda x,$ for some vector $x \neq 0,$ if and only if $\rho(a)x=\frac{1}{\lambda}x,$ i.e. if and only if $\frac{1}{\lambda}$ is an eigenvalue of $\rho(a).$ so if $\lambda_1, \cdots, \lambda_n$ are the eigenvalues of $\rho(a),$ then the

eigenvalues of $\rho(a^{-1})$ are: $\overline{\lambda_1}=\frac{1}{\lambda_1}, \cdots , \overline{\lambda_n}=\frac{1}{\lambda_n}.$ therefore: $\chi_{\rho}(a^{-1})=\sum \overline{\lambda_j}=\overline{\sum \lambda_j}=\overline{\chi_{\rho}(a)}.$
• May 3rd 2009, 06:20 PM
TreyD0g
how do you prove something is a representation?
• May 3rd 2009, 08:05 PM
NonCommAlg
Quote:

Originally Posted by TreyD0g

how do you prove something is a representation?

you need to prove that the map is a group homomorphism.
• May 3rd 2009, 08:13 PM
TreyD0g
do you know how to do that for the last problem i posted?
• May 3rd 2009, 08:53 PM
NonCommAlg
Quote:

Originally Posted by TreyD0g
do you know how to do that for the last problem i posted?

the reason that i haven't answered that question is that it's quite long and, well, you should do some of the questions yourself!

anyway, for part (a), you need to use the definition of $\pi$ to prove two things:

1) $\pi(g'),$ or as the notation in your problem $\pi_{g'},$ is an automorphism of $V$ for any $g' \in G.$ this proves that $\pi$ is well-defined.

2) for any $g_1,g_2 \in G: \ \ \pi(g_1g_2)=\pi(g_1) \pi(g_2),$ which will prove that $\pi$ is a homomorphism and thus a representation of $G.$
• May 4th 2009, 04:33 AM
TreyD0g
You're right. I should be doing these questions on my own, and i've spent hours focusing on this problem alone and haven't really gotten anywhere with it. I've even done a little research on Representation theory and i can't seem to find what i need to be able to solve this question.