# Find all matrices

1. Feb 7, 2005

### gunnar

I would appreciate if someone could help me with this one.

Matrix A=[1 0 ; 1 1]
I need to find all matrices that satisfy: AB=BA
How can I find them?

2. Feb 7, 2005

### Galileo

Try writing
$$B=\left(\begin{array}{cc}a & b \\ c&d \end{array}\right)$$

and check what the constraints on a,b,c and d are.

3. Feb 7, 2005

### gunnar

did that. And put A(inverse)* [a b;c d] = C*A(inverse)

4. Feb 7, 2005

### dextercioby

What does the inverse have to do with everything...?

Daniel.

5. Feb 7, 2005

### K.J.Healey

yea, just write out AB=BA using B=[a,b;c,d] and A=[1,0;1,1]
and multiply out both sides.