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A test for the finiteness of the number of discrete levels introduced into the gaps of a continuous spectrum by perturbations of a periodic potential. (English. Russian original) Zbl 0117.06004

Sov. Math., Dokl. 5, 689-692 (1964); translation from Dokl. Akad. Nauk SSSR 156, 515-518 (1964).

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