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The Hamiltonian in the question isDefineEach $I_j$ is a first integral, the functions involve disjoint canonical coordinate pairs and hence satisfy $\{I_j,I_k\}=0$, and their differentials are independent wherever every $z_j\ne 0$. Since $H={\sum }_j{I}_j$, these $n$ integrals prove Liouville integrability of the isotropic harmonic oscillator.

For constants $c_j>0$, the common level isso the regular invariant set isHamilton's equations give ${\dot{z}}_j=-i{z}_j$, so every angle evolves as ${\theta }_j(t)={\theta }_j(0)-t$. If some $c_j=0$, the corresponding circle collapses and the level is a lower-dimensional singular invariant torus.