Class that implements spherical potentials that are derived from power-law density models
NAME:
__init__
PURPOSE:
initialize a power-law-density potential
INPUT:
amp= amplitude to be applied to the potential (default: 1)
alpha= inner power
rc= cut-off radius
normalize= if True, normalize such that vc(1.,0.)=1., or, if given as a number, such that the force is this fraction of the force necessary to make vc(1.,0.)=1.
OUTPUT:
(none)
HISTORY:
2013-06-28 - Written - Bovy (IAS)