# Thread: hermitian matrix over C

1. ## hermitian matrix over C

Do anyone has an example of nxn hermitian matrix with complex entries which has repeated eigenvalues?
If can make the n as small as possible. Thank you

2. Originally Posted by guin
Do anyone has an example of nxn hermitian matrix with complex entries which has repeated eigenvalues?
If can make the n as small as possible. Thank you
How about $I_2$?

3. But if I would like to have entries such as 2+i or other which will involve the term i ?

4. The matrix $\begin{bmatrix}1&i&0\\ -i&1&0\\ 0&0&0\end{bmatrix}$ has a repeated eigenvalue 0.

Edit. Or if you want each eigenvalue to be repeated then you'll need a 4x4 matrix:

$\begin{bmatrix}1&i&0&0\\ -i&1&0&0\\ 0&0&1&i\\ 0&0&-i&1\end{bmatrix}.$

5. Originally Posted by guin
But if I would like to have entries such as 2+i or other which will involve the term i ?

If $A\in\mathbb{C}^{n\times n}$ is hermitian all its eigenvalues belong to $\mathbb{R}$ and is diagonalizable.

If you mean that $A$ has only one repeated eigenvalue $\lambda$ (therefore $n\geq 2$) then, $A$ is similar to $D=\lambda I_n$ that is, there exists $P\in\mathbb{C}^{n\times n}$ non singular such that $P^{-1}AP=\lambda I_n$ or equivalently $A=\lambda I_n$ so, $A$ has to be a real and scalar matrix.