- #1
QuarkCharmer
- 1,051
- 3
Homework Statement
This is from Lay, 2.1 #11 (the second part). Not a homework problem, just for fun.
Let A be the 3x3 matrix, A = [1,1,1; 1,2,3; 1,4,5]. Find a matrix B such that:
[tex]AB = BA[/tex]
where B is not the zero or identity matrix
Homework Equations
The Attempt at a Solution
Okay, so I know that typically, AB != BA, since matrix multiplication is non-commutative, but in some cases it can happen. What I did was make some 3x3 matrix B:
B = [a,b,c; d,e,f; g,h,i]
Then I wrote out AB = BA in matrix form, and solved both sides. Let's call AB matrix C, and BA matrix D. I set each entry in C to it's corresponding entry in D to form a system of 9 equations and 9 unknowns. I turned this into a 9x10 augmented matrix and attempted to rref it with my TI-89 but the resulting matrix is too big to display on the screen. I can scroll right and left, but I can't see anything below row 6.
Now, I am pretty confident that, if I did find a value for each entry in B (a,b,c,...,i) then it would form a matrix B such that AB = BA. But I don't think this is the right way to go about this problem at all. I can't believe they would expect me to solve 9 equations with 9 unknowns.
So my questions are:
1.) Would my approach have worked, say, if I computed it in mathematica.
and
2.) What is the proper approach to tackle this problem? I know there must be some way to go about this that is reasonable.Thanks!
-QC
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