@article{NUMA_1989__54_3_133314,
     author = {Georg Still},
     title = {Computable {Bounds} for {Eigenvalues} and {Eigenfunctions} of {Elliptic} {Differential} {Operators.}},
     journal = {Numerische Mathematik},
     pages = {201--224},
     publisher = {mathdoc},
     volume = {54},
     number = {3},
     year = {1989},
     zbl = {0662.65094},
     url = {http://geodesic.mathdoc.fr/item/NUMA_1989__54_3_133314/}
}