Analytic approach to perturbed Einstein ring with elliptical NFW lens
  model
  We investigate the validity of the approximate method to describe a strong
gravitational lensing which was extended by Alard on the basis of a
perturbative approach to an Einstein ring. Adopting an elliptical
Navarro-Frenk-White (NFW) lens model, we demonstrate how the approximate method
works, focusing on the shape of the image, the magnification, caustics, and the
critical line. Simplicity of the approximate method enables us to investigate
the lensing phenomena in an analytic way. We derive simple approximate formulas
which characterise a lens system near the Einstein ring.
