No evidence for the evolution of mass density power-law index $\gamma$
  from strong gravitational lensing observation
  In this paper, we consider the singular isothermal sphere lensing model that
has a spherically symmetric power-law mass distribution $\rho_{tot}(r)\sim
r^{-\gamma}$. We investigate whether the mass density power-law index $\gamma$
is cosmologically evolutionary by using the strong gravitational lensing (SGL)
observation, in combination with other cosmological observations. We also check
whether the constraint result of $\gamma$ is affected by the cosmological
model, by considering several simple dynamical dark energy models. We find that
the constraint on $\gamma$ is mainly decided by the SGL observation and
independent of the cosmological model, and we find no evidence for the
evolution of $\gamma$ from the SGL observation.
