An efficient positive potential-density pair expansion for modelling
  galaxies
  We present a novel positive potential-density pair expansion for modelling
galaxies, based on the Miyamoto-Nagai (MN) disc. By using three sets of such
discs, each one of them aligned along each symmetry axis, we are able to
reconstruct a broad range of potentials that correspond to density profiles
from exponential discs to 3D power law models with varying triaxiality
(henceforth simply "twisted" models). We increase the efficiency of our
expansion by allowing the scale length parameter of each disc to be negative.
We show that, for suitable priors on the scale length and height parameters,
these "MNn discs" have just one negative density minimum. This allows us to
ensure global positivity by demanding that the total density at the global
minimum is positive. We find that at better than 10\% accuracy in our density
reconstruction, we can represent a radial and vertical exponential disc over
$0.1-10$ scale lengths/heights with 4 MNn discs, an NFW profile over $0.1-10$
scale lengths with 4 MNn discs, and a twisted triaxial NFW profile with 3 MNn
discs per symmetry axis. Our expansion is efficient, fully analytic, and
well-suited to reproducing the density distribution and gravitational potential
of galaxies from discs to ellipsoids.
