A fast algorithm for estimating actions in triaxial potentials
  We present an approach to approximating rapidly the actions in a general
triaxial potential. The method is an extension of the axisymmetric approach
presented by Binney (2012), and operates by assuming that the true potential is
locally sufficiently close to some St\"ackel potential. The choice of St\"ackel
potential and associated ellipsoidal coordinates is tailored to each individual
input phase-space point. We investigate the accuracy of the method when
computing actions in a triaxial Navarro-Frenk-White potential. The speed of the
algorithm comes at the expense of large errors in the actions, particularly for
the box orbits. However, we show that the method can be used to recover the
observables of triaxial systems from given distribution functions to sufficient
accuracy for the Jeans equations to be satisfied. Consequently, such models
could be used to build models of external galaxies as well as triaxial
components of our own Galaxy. When more accurate actions are required, this
procedure can be combined with torus mapping to produce a fast convergent
scheme for action estimation.
