Analytic gas orbits in an arbitrary rotating galactic potential using
  the linear epicyclic approximation
  A code, Epic5, has been developed which computes, in the two-dimensional
case, the initially circular orbits of guiding centra in an arbitrary
axisymmetric potential with an arbitrary, weak perturbing potential in solid
body rotation. This perturbing potential is given by its Fourier expansion. The
analytic solution solves the linear epicyclic approximation of the equations of
motion. To simulate the motion of interstellar matter and to damp the Lindblad
resonances, we have in these equations introduced a friction which is
proportional to the deviation from circular velocity. The corotation resonance
is also damped by a special parameter. The program produces, in just a few
seconds, orbital and density maps, as well as line of sight velocity maps for a
chosen orientation of the galaxy.
  We test Epic5 by comparing its results with previous simulations and
observations from the literature, which gives satisfactory agreement. The aim
is that this program should be a useful complement to elaborate numerical
simulations. Particularly so are its abilities to quickly explore the parameter
space, to construct artificial galaxies, and to isolate various single agents
important for developing structure of interstellar matter in disc galaxies.
