Uniform Mesh Approximation to Nonsmooth Solutions of a Singularly Perturbed Convection-Diffusion Equation in a Rectangle

Here nonsmooth solutions of a differential equation are treated as solutions for which the compatibility conditions are riot required to hold at the corner points of the domain and hence corner singularities can occur. In the present paper, we drop the compatibility conditions at three of tire four vertices of a rectangle. At the remaining vertex, from which a characteristic (inclined) of the reduced equation issues, we impose compatibility conditions providing the C-3,C-lambda-smoothness of the desired solution in a neighborhood of that vertex as well as additional conditions leading to the smoothness of solutions of the reduced equation occurring in the regular component of the solution of the considered problem. Under our assumptions and for a sufficient smoothness of the coefficients of the equation and its right-hand side, we show that the classical five-point upwind approximation on a Shishkin piecewise uniform mesh preserves the accuracy specific for the smooth case; i.e., the mesh solution uniformly (with respect to a small parameter) converges in the L-infinity(h)-norm to the exact solution at the rate O(N-1 1n(2) N), where N is the number of mesh nodes in each of the coordinate directions.