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