For mixed boundary value problems of Poisson and/or Laplace's equations in regions of the Euclidean space En, n^2, finite-difference analogues are formulated such that the matrix of the resulting system is of positive type. Discretization errors are established in a manner to reveal the continuous dependence of the rate of convergence on the smoothness of the solution. Isolated data singularities and their application to exterior problems are also discussed. |