# Linear Algebra: characteristic polynomials and trace

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1. Nov 20, 2016

### MrsM

The question is : Is it true that two matrices with the same characteristic polynomials have the same trace?

I know that similar matrices have the same trace because they share the same eigenvalues, and I know that if two matrices have the same eigenvalues, they have the same trace. But I am struggling on linking the information in my head.

Your help is appreciated. Thanks :)

2. Nov 20, 2016

### PeroK

The trace is related to a particular coefficient in the characteristic polynomial. Can you see which one?

3. Nov 20, 2016

### Staff: Mentor

Hint: Develop the determinant in the characteristic polynomial along the first row and see which term involves the trace.

4. Nov 20, 2016

### MrsM

Dear Fresh_42 and Perok,

Thank you for responding, the question included no specific matrix. It is a question of concept. I suppose I could try to make one. But I was hoping I could answer using concepts.

Perok, I think it is for X^2, Y^2, Z^2 in a 3x3 matrix.

Fresh_42, I don't understand what you mean by developing the determinant along the first row to get the trace. I thought I added the eigenvalues to get the trace.

5. Nov 20, 2016

### PeroK

I'm not sure what you mean by that. Try to calculate the characteristic polynomial for a general $2 \times 2$ matrix. Using that can you do it for a general $3 \times 3$ matrix. Which coefficients of the polynomial seem to be following a pattern? Is one of them the trace? Can you generalise to $n \times n$ matrices using induction?

6. Nov 20, 2016

### Staff: Mentor

The characteristic polynomial is defined by $\det (A-\lambda \cdot I)$. And the determinant is the sum of the products along all even diagonals (from top left to bottom right) minus the sum of the products along all odd diagonals (from top right to bottom left). It can be computed with the Laplace formula which makes it accessible to proofs by induction.

Last edited: Nov 20, 2016
7. Nov 20, 2016

### MrsM

Fresh,
I don't understand, but I'm definitely trying. I've been up since 3am and have only completed 5 out of 30 questions on this assignment. I am looking at dozens of determinant calculations I have done over the past few weeks using Laplace. They are done correctly, but I am still struggling with how it relates to trace and the polynomial.

Perok, I did the following calculations per your suggestion, but fail to see the connection you are nudging me towards. please see below:
A is now
1, -3;
-3, 9;
the_Display_Matrix is now
1, -3;
-3, 9;
The determinant of the_Display_Matrix is
0;
the_Display_Matrix is now
0;
The trace of the_Display_Matrix is 0.
The trace of A is 10.
The determinant of A is
0;
B is now
1, 0, 1;
-2, 1, -3;
1, 2, 0;
The determinant of B is
1;
The trace of B is 2.
The eigenvalues of B are
0.17609282675202476 + 0 i
0.9119535866239881 + 2.201627508452126 i
0.9119535866239881 + -2.201627508452126 i
The characteristic polynomial of B is given by
det( B - z I ) = 1 + (-6) z + (2) z^2 + (-1) z^3

8. Nov 20, 2016

### PeroK

I have no idea what all that means. The characteristic equation of the general $2 \times 2$ matrix:
$$\left( \begin{array} \ a & b \\ c & d \end{array} \right)$$
Is:
$$\left| \begin{array} \ a-\lambda & b \\ c & d-\lambda \end{array} \right| = \lambda^2 - (a+d)\lambda + ad-bc$$
Do any of these coefficients look familiar?

9. Nov 20, 2016

### MrsM

I found this, because you said to look at a 2x2 and a 3x3, and see if they have something in common, and I am trying to see the correlation, but aside from that they both have something involving Trace in the middle, I don't. I appreciate your help, but at this point I have spent about 2 hours on a problem, and I should just move on to the next one. Thank you anyhow.

(3)
where is the matrix trace of and is its determinant.

Similarly, the characteristic polynomial of a matrix is

which can also be written explicitly in terms of traces as

Last edited by a moderator: Apr 17, 2017
10. Nov 20, 2016

### Staff: Mentor

Yes, and it generalizes to all matrices: the trace is the second highest coefficient, the determinant the lowest.

11. Nov 20, 2016

### MrsM

So does that mean since they have the same coefficients, the equation is the same? What if the values of x and y are different? Does that make the equation still the same?

And since the coefficients are the same, and the trace is represented by the second highest one, does that mean if two matrices have the same characteristic polynomial, the trace is the same? I would say yes, but my brain is so fried right now from going in circles... I just don't really know

12. Nov 20, 2016

### Staff: Mentor

Equality of two polynomials means equality of each coefficient. So if $\det(A-\lambda \cdot I)=\det(B-\lambda \cdot I)$ then $tr(A) = tr(B)$, as the traces are the second highest coefficient each.