# Abstract Algebra: Finding Conjugates

#### The_Iceflash

1. The problem statement, all variables and given/known data
Consider this group of six matrices:

Let G = {I, A, B, C, D, K}, Matrix Multiplication>

$$I =\begin{bmatrix}1 & 0\\0 & 1\end{bmatrix}$$ $$A =\begin{bmatrix}0 & 1\\1 & 0\end{bmatrix}$$ $$B =\begin{bmatrix}0 & 1\\-1 & -1\end{bmatrix}$$

$$C =\begin{bmatrix}-1 & -1\\0 & 1\end{bmatrix}$$ $$D =\begin{bmatrix}-1 & -1\\1 & 0\end{bmatrix}$$ $$K =\begin{bmatrix}1 & 0\\-1 & -1\end{bmatrix}$$

Operation Table for this group:

_|I A B C D K
I |I A B C D K
A|A I C B K D
B|B K D A I C
C|C D K I A B
D|D C I K B A
K|K B A D C I

Define $$f:G\rightarrow$$ $$\left\langle\(R^{*}, \bullet\right\rangle$$ by f(x) = det(x) for any Matrix x $$\in$$ G.

Finally, the question. Haha.

Find all the conjugates of A:
Find all the conjugates of B:

2. Relevant equations
N/A

3. The attempt at a solution

Now I know this isn't that hard of a concept and I understand what a conjugate is but I don't know how to find them. Any help on how I would go about finding them is greatly appreciated.

Last edited:
Related Calculus and Beyond Homework News on Phys.org

#### ABarrios

For conjugates of A,

G={I, A, B, C, D, K}

I^-1*A*I=A
B^-1*A*B=D*C=K
C^-1*A*C=C*B=K
D^-1*A*D=B*K=C
K^-1*A*K=K*D=C

So K and C are conjugates of A.

Do same procedure to find conjugates of B.

#### Martin Rattigan

Or given the fact that you've worked out the multiplication table, you can see by inspection that it's a nonabelian group of of order 6. (You don't need to check the associative law because that's taken care of by the elements being real matrices under multiplication.)

That means it's isomorphic to S3 and the conjugacy classes are
(i) the identity
(ii) the elements of order 2
(iii) the rest

The identity is I of course.
The elements of order 2 can be read off by finding I on the main diagonal, so A,C,K.
And the rest is B,D.

Of course you need to have proved that there is only one nonabelian group of order 6, and determined the conjugacy classes in S3 first.

#### The_Iceflash

Ok I understand that. Thanks. One question though, can A or B be considered a conjugate of either x-1Ax or x-1Bx respectively?

#### ABarrios

Yes. For take the identity element, I^-1*A*I=A So A is a conjugate of A, and likewise B is a conjugate of itself.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving