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A simple argument is used to show that when a spherical galaxy acts as a gravitational lens there must always be an odd number of images of the source and usually either (a) only one image of the source, or (b) three images, neglecting fine-scale (0.00001 arcsec) substructure, since the projected mass density decreases outward from the nucleus. It is shown how to calculate the amplification of each image, and the effect of different mass distributions for the lens galaxy is considered.
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Dyer et al. (1980) studied this question.
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