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RSKCorrespondence(YoungTableau, YoungTableau) not reconstructing biword correctly #36

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@seangrate

Taking the example from Wikipedia, we can verify that for the matrix $A$ below, the computed biword is correct.

i1 : A = matrix {{0,0,0,0,0,0,0},
                 {0,0,0,1,0,1,0},
                 {0,0,0,1,0,0,0},
                 {0,0,0,0,0,0,1},
                 {0,0,0,0,1,0,0},
                 {0,0,1,1,0,0,0},
                 {0,0,0,0,0,0,0},
                 {1,0,0,0,0,0,0}}

o1 = | 0 0 0 0 0 0 0 |
     | 0 0 0 1 0 1 0 |
     | 0 0 0 1 0 0 0 |
     | 0 0 0 0 0 0 1 |
     | 0 0 0 0 1 0 0 |
     | 0 0 1 1 0 0 0 |
     | 0 0 0 0 0 0 0 |
     | 1 0 0 0 0 0 0 |

              8       7
o1 : Matrix ZZ  <-- ZZ

i2 : biword A

o2 = | 2 2 3 4 5 6 6 8 |
     | 4 6 4 7 5 3 4 1 |

              2       8
o2 : Matrix ZZ  <-- ZZ

We also can verify the pair of semi-standard Young tableaux $(P,Q)$ that $A$ corresponds to.

i3 : (P, Q) = RSKCorrespondence A

o3 = (+---+---+---+, +---+---+---+)
      | 1 | 4 | 4 |  | 2 | 2 | 4 |
      +---+---+---+  +---+---+---+
      | 3 | 5 |      | 3 | 5 |   
      +---+---+      +---+---+
      | 4 | 7 |      | 6 | 6 |   
      +---+---+      +---+---+
      | 6 |          | 8 |       
      +---+          +---+       

o3 : Sequence

However, in the computation of the correspondence in the other direction (i.e., computing the image of $(P,Q)$), the biword of $A$ must be reconstructed while backtracking through the row-insertion algorithm. This reconstructed biword is not correct it should be biword A with the column order reversed. Below is the current reconstructed biword.

| 8 6 6 5 4 3 2 2 |
| 1 4 5 6 7 3 4 4 |

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