  The Rosensweig instability is the phenomenon that above a certain threshold
of a vertical magnetic field peaks appear on the free surface of a horizontal
layer of magnetic fluid. In contrast to almost all classical hydrodynamical
systems, the nonlinearities of the Rosensweig instability are entirely
triggered by the properties of a deformed and a priori unknown surface. The
resulting problems in defining an adjoint operator for such nonlinearities are
illustrated. The implications concerning amplitude equations for pattern
forming systems with a deformed surface are discussed.
