Hi, I've been working on a problem involving gravitational lens, and right now I'm trying to learn the formula to calculate magnification of light due to multiple gravitational lens. I've gotten to an equation that I've become stuck on. I've been using formulas from this paper: http://adsabs.harvard.edu/full/1992A&A...265....1S In particular, I'm don't understand equation 2.10, and 2.11, for the value of Ai where i is the nth 2x2 matrix. What would the Jacobian matrix look like? Thanks!