void evolve(int Nx, int Ny, double in[][Ny], double out[][Ny], double D, double dt) {

int i, j;

double laplacian;

for (i=1; i<Nx-1; i++) {

for (j=1; j<Ny-1; j++) {

laplacian = in[i+1][j] + in[i-1][j] + in[i][j+1] + in[i][j-1] - 4 * in[i][j];

out[i][j] = in[i][j] + D * dt * laplacian;

}

}

}