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The Poisson problem is a simple partial differentual equation (PDE) that is at the

core of many applications. The example is useful for desccribing the features

of MPI/OpenMP that can be used in solving partial differential equations and

of MPI/OpenMP/CUDA/OpenACC that can be used in solving partial differential equations and

other problems that involve decomposition across many processes.

Authors: Andrea Mignone   (mignone@to.infn.it)       ---> original version