# Similar matrices and main diagonal summation?

1. Apr 29, 2016

### zjohnson19

1. The problem statement, all variables and given/known data

True or False? If A is an n × n matrix, P is an n × n invertible matrix, and B = P −1AP, then
a11 + a22 + . . . + ann = b11 + b22 + . . . + bn
2. Relevant equations
Diagnolization, similar matrixes

3. The attempt at a solution
the question is asking if the summation of the main diagnols of A and B are the same. B is known to be similar to A since B = P^-1 AP. I can't find a counterexample, so I am assuming the summation of the diagnols of both are in fact equal, but this is hardly a proof. I know A and B share the same determinant, rank, and eigenvalues and rank, but I'm not sure how these relate to the main diagnol of the matrices.

2. Apr 29, 2016

### blue_leaf77

Are you familiar with the concept of trace of a matrix?

Last edited: Apr 29, 2016
3. Apr 29, 2016

### Twigg

The key is that A and B have the same characteristic polynomial. The sum of elements on the main diagonal (called the trace of the matrix) is the first-order coefficient of the characteristic polynomial. To prove that you can either use induction or start with the first order Vieta formula and prove that the trace equals the sum of the eigenvalues.

4. Apr 29, 2016

### blue_leaf77

The trace of a matrix $A$ is the sum of the diagonal elements, namely $\textrm{Tr }A = \sum_{i=1}^n A_{ii}$. This means the trace of a product of two matrices will be $\textrm{Tr }AB = \sum_{i=1}^n \sum_{j=1}^n A_{ij}B_{ji}$.

Since you have a product of three matrices, the the trace reads
$$\textrm{Tr }ABC = \sum_{i=1}^n \sum_{j=1}^n (AB)_{ij}C_{ji}$$
Now write $(AB)_{ij}$ in terms of the sum of products between the elements of $A$ and $B$.