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General Relativity gives that finitely many point masses between an observer and a light source create many images of the light source. Positions of these images are solutions of, where  r ( z ) is a rational function. We study the number of solutions toand, where  p ( z ) and  r ( z ) are polynomials and rational functions, respectively. Upper and lower bounds were previously obtained by Khavinson–Świa̧tek, Khavinson–Neumann, and Petters. Between these bounds, we show that any number of simple zeros allowed by the Argument Principle occurs and nothing else occurs, off of a proper real algebraic set. Ifdescribes an  n -point gravitational lens, we determine the possible numbers of generic images.

Counting Zeros of Harmonic Rational Functions and its Application to Gravitational Lensing