1. Bramble, J. H.: Error estimates for difference methods in forced vibration problems. SIAM Series B3, No. 1, 1–12 (1966).
2. —— andB. E. Hubbard: On the formulation of finite difference analogues of the Dirichlet problem for Poisson's equation. Num. Math.4, 313–327 (1962).
3. —— —— A priori bounds on the discretization error in the numerical solution of the Dirichlet problem. Contributions to Diff. Eq. V2, 229–252 (1963).
4. —— —— New monotone type approximations for elliptic problems. Math. Comp. V18, 349–367 (1964).
5. —— —— Approximations of derivatives by finite difference methods in elliptic boundary value problems. Contributions to Diff. Eq. V3, No. 4, 399–410 (1964).