Geodesic
The negative part of the spectrum of the operator $(-1)^m\Delta^mu+qu$
Matematičeskie zametki, Tome 9 (1971) no. 6, pp. 677-686.

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Exact criteria are obtained for the presence or absence of a negative part of spectra of operators generated by the differential operator $l(u)=(-1)^m\Delta^mu+qu$ 
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     author = {V. G. Mishin},
     title = {The negative part of the spectrum of the operator $(-1)^m\Delta^mu+qu$},
     journal = {Matemati\v{c}eskie zametki},
     pages = {677--686},
     publisher = {mathdoc},
     volume = {9},
     number = {6},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1971_9_6_a7/}
}
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V. G. Mishin. The negative part of the spectrum of the operator $(-1)^m\Delta^mu+qu$. Matematičeskie zametki, Tome 9 (1971) no. 6, pp. 677-686. http://geodesic.mathdoc.fr/item/MZM_1971_9_6_a7/