# Elementary Linear Algebra - Similar Matrices and Rank

1. Dec 1, 2011

### Rockoz

1. The problem statement, all variables and given/known data
Suppose matrices A and B are similar. Explain why they have the same rank.

2. Relevant equations

3. The attempt at a solution
So if A and B are similar, then there is some invertible matrix P such that B = P^-1AP. I have been trying to find some way to relate rank(A) to rank(P^-1AP) but I can't figure it out. I feel like maybe i'm missing some intuition about this. This comes at the end of a section about linear transformations.

Thank you to anyone who can help.

2. Dec 1, 2011

### HallsofIvy

Staff Emeritus
What is the definition of "rank"?

3. Dec 1, 2011

### Rockoz

"The dimension of the row (or column) space of a matrix is called the rank of A and is denoted by rank(A)."

Thinking about this I recall that the rank of a linear transformation is equal to the rank of the matrix for T (where T is defined by T(x) = Ax). So if A and B both represent the same linear transformation but with respect to different bases, then they will have the same rank. This relationship is described by them being similar. P and P^-1 are the transition matrices between the bases.

Is my thinking headed in the right direction? I still feel unclear about this. Is there a way to discuss this without reference to transformations and just from the definition of being similar?

The rank of A is the dimension of the image of A. So take a basis of the image of A. Can you maje this into a basis of the image of $PAP^{-1}$??