@article{NUMA_1965__7_131679,
     author = {H.B. KELLER},
     title = {On the {Accuracy} of {Finite} {Difference} {Approximations} to the {Eigenvalues} of {Differential} and {Integral} {Operators.}},
     journal = {Numerische Mathematik},
     pages = {412--419},
     publisher = {mathdoc},
     volume = {7},
     year = {1965},
     zbl = {0135.37702},
     url = {http://geodesic.mathdoc.fr/item/NUMA_1965__7_131679/}
}