# Robinson-Schensted algorithm

robinson_schensted

## Definitions

The Robinson-Schensted (RS) algorithm is a combinatorial algorithm on Young tableaux.

The basic operation is the insertion of a positive integer to a Young tableau to form a new tableau.

### Row insertion

The definition found in most texts is the row insertion, which we define now.

Say we want to insert $$k_1 = k$$ into $$T$$ to form a new tableau $$\tilde T$$. Starting from the first row of $$T$$, if $$k$$ is no less than all entries in the first row, then it is appended at the end of said row and the resulting tableau is the new tableau $$\tilde T$$, otherwise $$k$$ displaces the leftmost number in that row that is greater than $$k$$, and insert the displaced number $$k_2$$ into the second row. Then $$k_2$$ is either appended to the end of the second row and the algorithm halts and the resulting tableau is $$\tilde T$$, or it displaces $$k_3$$, the leftmost number in the second row that is greater than $$k_2$$.

...

In general $$k_j$$ is either appended to the $$j$$th row and the algorithm halts and produces the resulting tableau, or it displaces $$k_{j + 1}$$ the leftmost number in the $$j$$th row that is greater than $$k_j$$.

The process runs until the algorithm halts.

Using the correspondence between the Young tableaux and the Gelfand-Tsetlin patterns, the RS algorithm is a map that takes a Gelfand-Tsetlin pattern and a positive integer as input, and produces a new GT pattern as the output. Here is a sample Python code for the algorithm. Note EnlargeGT is a helper function that generate an equivalent GT of bigger size if the inserted number $$k$$ is big.

def EnlargeGT(Lambda, k):
from copy import deepcopy
NewLambda = deepcopy(Lambda)
l = len(Lambda)
if l < k:
for i in range(l, k):
if l == 0:
NewLambda += [[0] * (i + 1)]
else:
NewLambda += [Lambda[l - 1] + [0] * (i - l + 1)]
return NewLambda

def RowInsertionGT(Lambda, k):
NewLambda = EnlargeGT(Lambda, k)
l = len(NewLambda)
j = 1
for i in range(k, l):
NewLambda[i - 1][j - 1] += 1
if NewLambda[i - 1][j - 1] <= NewLambda[i][j - 1]:
j += 1
NewLambda[l - 1][j - 1] += 1
return NewLambda


Recall the functions GT2Tableau and Tableau2GT in correspondence between the Young tableaux and the Gelfand-Tsetlin patterns, the Python code for the row insertion applied to a Young tableau is RowInsertionGT "conjugated" with the conversions:

def RowInsertion(T, k):
return GT2Tableau(RowInsertionGT(Tableau2GT(T), k))


### Column insertion

Column insertion has the same definition as the row insertion, except that

1. The words 'row' and 'column' are interchanged
2. The number to be inserted in each column displaces the smallest number that is greater than or equal to itself, rather than 'greater than'.

Similarly it can be conveniently described in the language of GT patterns, and here are the sample codes, where we reuse EnlargeGT from the row insertion code:

def ColInsertionGT(Lambda, k):
NewLambda = EnlargeGT(Lambda, k)
l = len(NewLambda)
j = k
for i in range(k, l + 1):
while j > 1 and NewLambda[i - 2][j - 2] == NewLambda[i - 1][j - 1]:
j -= 1
NewLambda[i - 1][j - 1] += 1
return NewLambda

def ColInsertion(T, k):
return GT2Tableau(ColInsertionGT(Tableau2GT(T), k))


### RS algorithm

The RS algorithm takes a word $$w = w_1w_2...w_n$$ and inserts $$w_1$$ to the empty tableau to obtain $$T(1)$$, and inserts $$w_2$$ to $$T(1)$$ to obtain $$T(2)$$, and so on and so forth. The output of the algorithm taking $$w$$ is $$T = T(n)$$. This applies to both row and column insertions.