Invariant mass approach to the spiral structure of galaxies

Following an earlier suggestion that spiral density waves in galaxies primarily are a response phenomenon to the increase with time of a central quadrupole moment. The object of this note is to describe a simple procedure adequate for estimating the shape of the density response in a stellar disk for given structures of the background and perturbative gravitational fields, respectively, under the premises of first order epicyclic and adiabatic approximation.