Thanks for the article and the access to the magazine. You wrote that the Helmholtz motion shows identifying sign of a non-linear system. Is there a proof which shows that the physical correct solution cannot be obtained as a superposition of (sinus) modes obeying a linear wave equations?

## Helmholtz motion

Thanks for the article and the access to the magazine. You wrote that the Helmholtz motion shows identifying sign of a non-linear system. Is there a proof which shows that the physical correct solution cannot be obtained as a superposition of (sinus) modes obeying a linear wave equations?