Solution (source code)

= Solution

The <minimal polynomial> factors over $\mathbb C$ as $(t-3)(t-3i)(t+3i)$ with distinct roots. The <diagonalizability criterion using the minimal polynomial> therefore makes $\alpha$ diagonalizable. Its <characteristic polynomial> gives multiplicities two, one and one, so its <Jordan normal form> is
$$
\boxed{\operatorname{diag}(3,3,3i,-3i),}
$$
up to reordering the one-dimensional <Jordan blocks>.