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RSK interchanges the insertion and recording tableaux under inversion

Statement

Let n≥0 and let w=(w1,…,wn) be a permutation of {1,…,n} with RSK pair (P(w),Q(w)) (The Robinson-Schensted correspondence). Let w−1=(p1,…,pn) be the inverse permutation, where pi is the position of i in w (so wpi=i). Then P(w−1)=Q(w)andQ(w−1)=P(w). In particular, identifying σ∈Sn with the word (σ(0)+1,…,σ(n−1)+1) via The finite symmetric group Sn, one-line notation, and cycle notation, the RSK pair of σ−1 is (Q,P).

Facts & Assumptions

Given: An integer n≥0, a word w=(w1,…,wn) of pairwise distinct real numbers with {w1,…,wn}={1,…,n}, its RSK pair (P(w),Q(w)), and the inverse word w−1=(p1,…,pn) with wpi=i.

[L1]

The Robinson-Schensted map is a bijection from the set Xn of such words onto the set of pairs (P,Q) of standard tableaux of a common shape λ⊢n; equivalently, two words with the same RSK pair are equal, and every such pair occurs (The Robinson-Schensted correspondence).

[L2]

For a lexicographically ordered two-line array (u;v): (a) construction A (insert vk, write the label uk in the new box) produces a pair of semistandard tableaux of common shape, and the correspondence with lexicographically ordered arrays is a bijection; (b) the lexicographically ordered rearrangement of the transposed array (v;u) corresponds to (Q,P); (c) if uk=k for all k then construction A is exactly the row-insertion construction of The Robinson-Schensted correspondence and produces its RSK pair (The RSK correspondence for two-line arrays).

[L3]

A two-line array is a pair of finite lists of positive integers whose columns are in nondecreasing lexicographic order; the top line u is strictly increasing exactly when its entries are distinct and increasing (The RSK correspondence for two-line arrays).

[L4]

Sn acts on {0,1,…,n−1}, one-line notation and cycles are as in The finite symmetric group Sn, one-line notation, and cycle notation; a standard tableau is a filling of a Young diagram by distinct integers increasing along rows and columns (Tableaux and standard tableaux).

Proof

technique · direct
1.1L3given

The two-line array A:=((1,2,…,n);(w1,…,wn)) is lexicographically ordered, because its top line is strictly increasing; its top line has distinct entries and its columns are the pairs (k,wk) for k=1,…,n.

1.2L2L1given

Construction A applied to A inserts vk=wk at step k and writes uk=k in the box added at step k; by L2 the resulting pair is the RSK pair of the word (w1,…,wn), namely (P(w),Q(w)).

1.3L3givenalgebra

The transposed array of A is ((w1,…,wn);(1,2,…,n)), whose columns are the pairs (wk,k); since w is a permutation of {1,…,n}, each value i occurs exactly once among the first coordinates, at the index k=pi with wpi=i, so the lexicographically ordered rearrangement of these columns is the array A−1:=((1,2,…,n);(p1,…,pn)).

1.4L4givenalgebra

For the group-theoretic form, let σ∈Sn have one-line form [σ(0),…,σ(n−1)] and let w=(σ(0)+1,…,σ(n−1)+1) be the associated word; then for each i∈{1,…,n} the position pi of i in w satisfies pi=σ−1(i−1)+1, because wσ−1(i−1)+1=σ(σ−1(i−1))+1=i; hence the word associated with σ−1 is exactly w−1=(p1,…,pn).

2.1L2step 1.2step 1.3

By L2 the array A−1 corresponds under construction A to (Q(w),P(w)): it is the lexicographically ordered rearrangement of the transpose of the array A of step 1.2, whose pair is (P(w),Q(w)).

2.2L2L1givenstep 1.3

Construction A applied to A−1 inserts p1,…,pn and writes the labels 1,…,n, so by L2 it produces the RSK pair (P(w−1),Q(w−1)) of the word w−1=(p1,…,pn).

3.1L2step 2.1step 2.2

By L2 the correspondence between lexicographically ordered two-line arrays and pairs is bijective, and the array A−1 corresponds to exactly one pair; by steps 2.1 and 2.2 this pair is both (Q(w),P(w)) and (P(w−1),Q(w−1)), so P(w−1)=Q(w) and Q(w−1)=P(w).

4.1step 3.1step 1.4∎

Applying step 3.1 to the permutation σ identified with w gives (P(σ−1),Q(σ−1))=(Q(σ),P(σ)), which is the stated group-theoretic form.

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