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Matrix dx(Matrix A) {
  
  int M = A.getRowDimension();   
  int N = A.getColumnDimension();
  
  Matrix R = new Matrix(M, N);
  
  for (int y = 0; y < M; y++)
    for (int x = 1; x < N-1; x++) {
        double d1 = A.get(y,x-1);
        double d2 = A.get(y,x+1);
        double r = (d2 - d1)/2.0;
        R.set(y,x,r);
    }

  return R;
}