We discuss the (in)stability of solitary waves for a quasi-linear Schrödinger equation. The equation contains a quasi-linear term, responsible for a saturation effect, as well as a power nonlinearity. For different exponents of the nonlinearity, we determine analytically the asymptotic behavior of the \(L^2\) -mass of the solution as a function of the frequency close to the critical frequencies, which leads to natural conjectures concerning their stability. Depending on the exponent and the dimension, we expect all solitary waves to be stable, or the emergence of both a stable and an unstable branch of solutions. We investigate our conjectures numerically and find compatible results both for the mass–energy relation and the dynamics. We observe that perturbations of solitary waves on the unstable branch may converge dynamically to the stable solution of a similar mass, or disperse. More general initial conditions show a similar behavior.
