One of the way in which this can be done is by using the classical Laplace-Lagrange secular theory, first developed to study the evolution of planetary orbits. Within this framework, interacting bodies are smeared into massive rings, and the resulting perturbations -- expressed mathematically in what is known as the disturbing function -- is then written as a power series in orbital eccentricities and inclinations and a Fourier series in the orbital angles. Thus, an astrophysical disk may, in principle, be modelled as a continuum of perturbers, i.e., with very large number of rings, each interacting with the others as per the classical disturbing function. This approach, however, is ill-posed from a mathematical point of view when applied to self-gravitating disks, since it would result in singularities. This singularity stems from the fact that the gravitational potential diverges at null interparticle separations. This issue is often overcome in the literature by making use of softened forms of gravity, i.e., by spatially smoothing the Newtonian point-mass potential. This is essentially done by introducing a small, but non-zero, softening length into the calculations, rendering the force between two rings finite  -- rather than infinite -- at points of orbit crossings.