Instability induced by symmetry reduction

We demonstrate that instabilities in a Hamiltonian system can occur via deformations that reduce the symmetry of the system. The movement of eigenvalues at an equilibrium point of a family of Hamiltonian systems is constrained by the symmetry type of the system. If deformations of a family change the symmetry type, then instabilities can appear at multiple eigenvalues that produce large amplitude changes in the system dynamics. We illustrate this phenomenon in the context of a low-dimensional Hamiltonian normal form, and then analyze the instability of a vortex filament in a strain field.