Geodesic
On conditions that the spectra of certain operators be discrete
Matematičeskie zametki, Tome 11 (1972) no. 3, pp. 233-240.

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Certain necessary and sufficient conditions for the discreteness of the spectrum of an operator are obtained in terms of its symbol and also in terms of its Wick and anti-Wick symbol. 
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     author = {M. A. Shubin},
     title = {On conditions that the spectra of certain operators be discrete},
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     number = {3},
     year = {1972},
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M. A. Shubin. On conditions that the spectra of certain operators be discrete. Matematičeskie zametki, Tome 11 (1972) no. 3, pp. 233-240. http://geodesic.mathdoc.fr/item/MZM_1972_11_3_a0/