double localNorm = 0;
        for (i = 1; i < mx1; i++)
        {
            for (j = 1; j < my1; j++)
            {
                double df = fabs(newf[i][j] - f[i][j]);
                if (df > localNorm)
                    localNorm = df;
            }
        }
        double globalNorm = 0;
        MPI_Allreduce(&localNorm, &globalNorm, 1, MPI_DOUBLE, MPI_MAX, MPI_COMM_WORLD);