# Diagonalized 3x3 Matrix

1. Dec 18, 2008

### jkeatin

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

quick question, if there is a 3x3 matrix which has exactly 3 distinct eigenvalues why must it be diagonalizable?

2. Relevant equations

3. The attempt at a solution

2. Dec 18, 2008

### Vid

How is diagolization related to the dimension of the eigenspace? Why would distinct eigenvalues let you satisfy this dimension criteria?

3. Dec 18, 2008

### jkeatin

well since its a 3x3 matrix, the most eigenvalues it could have is 3? so since there is 3 unique eigenvalues, then it is definitely diagonalizable?

4. Dec 18, 2008

### Vid

You need two theorems to show this is true and I hinted at them in my post.

5. Dec 18, 2008

### HallsofIvy

Staff Emeritus
I think it is better to think in terms of linear transformations- any linear transformation can be represented as a matrix in a given basis: Apply the linear tranformation to each of the basis vectors in turn, the write the result as a linear combination of the basis vectors- the coefficients are the columns of the matrix.

The three distinct eigenvalues must have 3 independent eigenvectors. Using those eigenvectors as a basis for the vector space, the linear operator is represented by a diagonal matrix with the eigenvalues on the diagonal.

Equivalently, if A is a matrix with three distinct eigenvalues, B is the matrix having those three eigenvectors as columns, then B-1AB is the diagonal matrix having the eigenvalues on the diagonal.

By the way, having three independent eigenvectors is a necessary condition for a matrix to be diagonalizable. Having three distinct eigenvalues is not necessary.

6. Dec 18, 2008

### jkeatin

so what if a 3x3 matrix only has two eigenvalues, does that mean its not able to be diagonalized?

7. Dec 18, 2008

### Vid

No, reread what Halls wrote. One of the eigenvalues could have a two dimensional eigenspace.

8. Dec 18, 2008

### jkeatin

Thanks guys, I think its making sense, we will see on finals week lol.