Green’s Function Estimates for a ConvectionDiffusion Problem in Three Dimensions

Linear singularly perturbed convection-diffusion problems with characteristic layers are considered in three dimensions. We demonstrate the sharpness of our recently obtained upper bounds for the associated Green’s function and its deriva-tives in the L1 norm. For this, in this paper we establish the corresponding lower bounds. Both upper and lower bounds explicitly show any dependence on the sin-gular perturbation parameter