Geodesic
The spectrum of an elliptic operator of second order
Matematičeskie zametki, Tome 20 (1976) no. 3, pp. 351-358

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Under minimal requirements on the coefficients and the boundary of the domain it is proved that the spectrum of the first boundary-value problem for an elliptic operator of second order always lies in the half-plane $\lambda'\le\operatorname{Re}\lambda$, where $\lambda'$ is the leading eigenvalue to which there corresponds a nonnegative eigenfunction. On the line $\operatorname{Re}\lambda=\lambda'$, there are no other points of the spectrum. 
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     author = {T. M. Kerimov and V. A. Kondrat'ev},
     title = {The spectrum of an elliptic operator of second order},
     journal = {Matemati\v{c}eskie zametki},
     pages = {351--358},
     publisher = {mathdoc},
     volume = {20},
     number = {3},
     year = {1976},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1976_20_3_a5/}
}
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T. M. Kerimov; V. A. Kondrat'ev. The spectrum of an elliptic operator of second order. Matematičeskie zametki, Tome 20 (1976) no. 3, pp. 351-358. http://geodesic.mathdoc.fr/item/MZM_1976_20_3_a5/