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The properties of an electron in a disordered solid are discussed with use of a matrix nonlinear model first introduced by Wegner and Sch\"afer. The model is defined on the noncompact space O (M, M) O (M) (M) where M is the number of replicas. This noncompact symmetry represents the assential physics of the problem. It is found that all states are localized in two dimensions; above two dimensions for weak disorder there are mobility edges, but these merge above a critical amount of disorder and all states become localized.
Houghton et al. (Mon,) studied this question.