1. The problem statement, all variables and given/known data I'm studying about gravitational lensing and got stuck on an article: http://arxiv.org/pdf/0708.2684v1.pdf where the equation 2.1 is abit strange to me. The authour uses the reference http://arxiv.org/pdf/astro-ph/9703103v1.pdf for deriving the equation. According to me, the sum on equation is extremly similar to Gravitational Potential. 2. Relevant equations Equation 1 has the form w = z - Ʃsigma/(bar(z)-bar(zj)) and the gravitational potential is given in http://en.wikipedia.org/wiki/Gravitational_potential as Ʃ-Gmi/(x-xi) 3. The attempt at a solution I think that the complex sum in 2.1 is the same as gravitational potential. My question is if this is true? Or do I need to do some more research about the derivation?