Geodesic
The lower bound of the least negative eigenvalue $-\lambda^2$ of the equation $\Delta u+[f(r)-\lambda^2]u=0$
Žurnal vyčislitelʹnoj matematiki i matematičeskoj fiziki, Tome 2 (1962) no. 4, pp. 705-706 
O. Ya. Savchenko. The lower bound of the least negative eigenvalue $-\lambda^2$ of the equation $\Delta u+[f(r)-\lambda^2]u=0$. Žurnal vyčislitelʹnoj matematiki i matematičeskoj fiziki, Tome 2 (1962) no. 4, pp. 705-706. http://geodesic.mathdoc.fr/item/ZVMMF_1962_2_4_a13/
@article{ZVMMF_1962_2_4_a13,
     author = {O. Ya. Savchenko},
     title = {The lower bound of the least negative eigenvalue $-\lambda^2$ of the equation $\Delta u+[f(r)-\lambda^2]u=0$},
     journal = {\v{Z}urnal vy\v{c}islitelʹnoj matematiki i matemati\v{c}eskoj fiziki},
     pages = {705--706},
     year = {1962},
     volume = {2},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZVMMF_1962_2_4_a13/}
}
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