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C. Hunter (FSU), N.W. Evans (Oxford)
We present analytical results for the four and five image systems produced by lenses for which the lensing potential is of power-law form and is stratified on similar concentric ellipses. There may or may not be a central core. We evaluate sums of the signed magnifications, and sums of the products of the signed magnifications with positive and/or negative powers of the position coordinates of the images. All are expressed in terms of the parameters of the potential, a constant external shear, and the position of the source. Because all of these sums can be constructed from observables, our analytical results can be applied to data to discover if they are consistent with our models, and, if so, to estimate the model parameters. Some such applications are discussed.