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CONTENTS 1. Introduction. Choice of Model 549 2. Formulation of the Problem. General Considerations 552 3. The Behavior of the Spectral Density Near the True Boundary of the Spectrum 553 4. The Behavior of the Spectral Density Near Impurity Levels. The Theory of the Broadening of Levels with Concentration 555 5. Structure of the Spectrum Near the Unperturbed Boundary, E0g 561 6. The Structure of the Entire Spectrum 566 7. Some Other Models of Disordered Structures 571 Cited Literature 573
I.M. Lifshitz (Fri,) studied this question.