  We model a massless viscous disk using Smoothed Particle Hydrodynamics (SPH)
and note that it evolves according to the Lynden-Bell \& Pringle theory (1974)
until a non-axisymmetric instability develops at the inner edge of the disk.
This instability may have the same origin as the instability of initially
axisymmetric viscous disks discussed by Lyubarskij et al. (1994). To clarify
the evolution we evolved single and double rings of particles. It is actually
inconsistent with the SPH scheme to set up a single ring as an initial
condition because SPH assumes a smoothed initial state. As would be expected
from an SPH simulation, the ring rapidly breaks up into a band. We analyse the
stability of the ring and show that the predictions are confirmed by the
simulation.
