A  basis for the homogeneous polynomials annihilated by the operators

                          4     4     4    3     3     3
                        D1  + D2  + D3 , D1  + D2  + D3

                                 of degree<=4.

                                      is:

   2   2    2   2    2   2          2       2       2             3        3
{x1  x2 , x1  x3 , x2  x3 , x1 x2 x3 , x1 x2  x3, x1  x2 x3, x1 x3  - x1 x2 ,

         3     3       3        3       4          3       3        4
    x2 x3  - x1  x2, x2  x3 - x1  x3, x2  + 4 x1 x2  - 4 x1  x2 - x1 ,

      4          3       3        4
    x3  + 4 x1 x2  - 4 x1  x3 - x1 }

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