Solution
= Solution
For generic $\mu^2\ne\lambda^2$, the symmetry is $O(1)\times O(2)$: an independent sign flip of $\phi_1$ and orthogonal transformations of $(\phi_2,\phi_3)$. When $\mu^2=\lambda^2$, it is enhanced to $O(3)$.
= Solution
For generic $\mu^2\ne\lambda^2$, the symmetry is $O(1)\times O(2)$: an independent sign flip of $\phi_1$ and orthogonal transformations of $(\phi_2,\phi_3)$. When $\mu^2=\lambda^2$, it is enhanced to $O(3)$.