On the stability of the shear-free condition
  The evolution equation for the shear is reobtained for a spherically
symmetric anisotropic, viscous dissipative fluid distribution, which allows us
to investigate conditions for the stability of the shear-free condition. The
specific case of geodesic fluids is considered in detail, showing that the
shear-free condition, in this particular case, may be unstable, the departure
from the shear-free condition being controlled by the expansion scalar and a
single scalar function defined in terms of the anisotropy of the pressure, the
shear viscosity and the Weyl tensor or, alternatively, in terms of the
anisotropy of the pressure, the dissipative variables and the energy density
inhomogeneity.
