Let $SO(3)$ act on the space of $3 \times 3$ real matrices by conjugation. How can I decompose the space of matrices into the sum on minimal invariant subspaces and figure out what they are isomorphic to?
I am familiar with the irreducible representations of $SU(2)$ and how they give the irreducible representations of $SO(3)$. I don't see how to relate this notion to the minimal invariant subspaces.
Hints:
Some simple observations:
1) Due to the cyclic property of the trace $\mathop{\rm tr} M = \mathop{\rm tr} (O M O^T)$ for any matrix $M$ and $O \in SO(3)$.
2) Given a symmetric matrix $S=S^T$, the conjugated matrix $S' = O S O^T$ is also symmetric.
3) Statement 2) also holds when replacing symmetric with antisymmetric.
4) $1+3 + 5 = 9$
I hope this helps... (alternatively you can calculate the character as Qiaochu suggested)