Before revision:  Solvable \rho's are given by \rho = - \cos \left( \pi{k} \right), where k \in Z^+ \cup \{ -1 \}.
After revision:  Solvable \rho's are given by \rho = - \cos \left( \pi{k} \right), where k \in Z^+ \cup \{ +\infty \}.