On the Accuracy of Finite Difference Approximations to the Eigenvalues of Differential and Integral Operators. H.B. KELLER — 1965 Numerische Mathematik
Difference Approximations for Singular Perturbations of Systems of Ordinary Differential Equations. L.R. Abrahamsson; H.B. Keller; H.O. Kreiss — 1974 Numerische Mathematik