A simple method to construct exact density-potential pairs from a homeoidal expansion

We start from a study of the density- potential relation for classical homeoids in terms of an asymptotic expansion for small deviations from spherical symmetry. We then show that such expansion is a useful device that allows us to construct a variety of exact density- potential pairs with spheroidal, toroidal, or triaxial shapes for which the deviation from spherical symmetry is finite. As concrete analytical applications, we describe: ( 1) The construction of a family of toroidal axisymmetric density- potential pairs one of which is associated with a perfectly flat rotation curve ( for a member of this family, the supporting two- integral phase- space distribution function is obtained in closed form); ( 2) The determination of the aperture velocity dispersion in a wide class of two- integral axisymmetric models not stratified on homeoids with central black hole, which may be useful for the discussion of the dynamical contributions to the characteristics of the Fundamental Plane of early- type galaxies; and ( 3) For such class of models, the construction of the v/sigma-ellipticity relation, often considered to assess the role of rotation in the structure of elliptical galaxies.