2
$\begingroup$

How do you find the eigenvalues (hence the eigenvectors too) of a matrix with complex bits like this:

$\hat{H}=\epsilon \begin{vmatrix} 0&i&0 \\\\ -i&0&0 \\\\ 0&0&-i \end{vmatrix}$

With $\epsilon$ real. How do you find the eigenvalues (hence the eigenvectors too) of a matrix with complex bits like this:

$\hat{H}=\epsilon \begin{vmatrix} 0&i&0 \\\\ -i&0&0 \\\\ 0&0&-i \end{vmatrix}$

With $\epsilon$ real.

I get so far as this:

$|\hat{H}-\lambda{}I | = \epsilon \begin{vmatrix} -\lambda&i&0 \\\\ -i&0-\lambda&0 \\\\ 0&0&-i-\lambda \end{vmatrix} =0$

$|\hat{H}-\lambda{}I | =\epsilon(-\lambda(-\lambda(i+\lambda))+i(0-i(i+\lambda))$

$=\epsilon(\lambda^2 (i+\lambda)+(i+\lambda))=0$

Then I am not sure what to do next. I think the $\epsilon$ cancels and I can do this:

$= \lambda^3+\lambda^2i+i+\lambda=0$

2 Answers 2

1

You have a factor $\lambda +i$, which gives $-1$ as eigenvalue. Then you have to solve $\lambda^2+1=0$, which gives $i$ again and $-i$.

1

$|xI-H|=\epsilon(x+i)(x^2-1)=\epsilon(x+i)(x-1)(x+1)$

So the eignevalues are $\,-i\,,\,\pm 1\,$. The matrix is thus diagonalizable

  • 1
    Since a matrix is diagonalizable iff there exists a basis of its eigenvectors, and since eigenvectors belonging to different eigenvalues are linearly independent, a $\,3\times 3\,$ matrix with 3 different eigenvalues is diagonalizable.2012-11-04