Can continuous density profile correctly describe strong gravitational
  lensing?
  Strong gravitational lensing is an important tool to probe the universe. In
the theoretical analysis of gravitational lensing, it is assumed that
continuous density profile can correctly describe the lens galaxy. But in fact
this assumption has never been rigorously tested. In this paper, we discuss
this issue, and point out that if we use discrete density profile to model the
lens galaxy, then the position of the images does not change, but the
magnification will be increased. Strongly lensed gravitational waves could test
this conclusion in the future.
