
Solitary waves are critical points of Hamiltonians in the constrained variational principle. Spectral
stability of solitary waves follows from the convexity of the second variation of the constrained Lyapunov
functional. Spectral instabilities occur when the second variation is not signdefinite. Alternatively,
spectral stability and instability theory is developed from analysis of eigenvalues of nonselfadjoint
operators, which are compositions of symplectic and selfadjoint operators.
Using the Hamiltonian formalism of the nonlinear PDEs, we study eigenvalues of the spectral stability problem
for solitary waves in the NLS, Dirac, and KdVtype equations. Using the spectral methods in Hilbert spaces,
we find rigorous bounds on the number of unstable eigenvalues in the stability problems. Using dynamical
system methods, we derive normal forms for nonlinear instabilityinduced dynamics of solitary waves.
Applications include optical solitons, BoseEinstein condensates, photonic materials, and water wave solitons.

