# RS algorithms as type-A Pitman's transforms

pitman_transform robinson_schensted

In this article we derive the type-A pitman's transform from the Robinson-Schensted algorithms. We start with the column insertion version, which can be found in [{oconnell03b}] and [{biane-bougerol-oconnell05}]. Then we show the row insertion version, which is symmetric and quite similar to the column insertion version.

## Rank-one

We start with the RS algorithm with column insertion that takes a stream of ones and twos as input: $$w \in \{1, 2\}^\infty$$.

We write

\begin{align} S_1(n) = \sum_{i = 1 : n} 1_{w_i = 1}\\ S_2(n) = \sum_{i = 1 : n} 1_{w_i = 2} \end{align}

to be number of ones and twos up to time $$n$$. Then $$(S_1, S_2)$$ can be regarded as a two-dimensional walk (or path) going by $$(1, 0)$$ or $$(0, 1)$$ each time.

Recall the RS algorithm with column insertion applied to $$w$$:

1. If $$w_n = 1$$ then
1. $$\lambda^1_1(n) = \lambda^1_1(n - 1) + 1$$
2. $$\lambda^2_1(n) = \lambda^2_1(n - 1) + 1$$
2. Else (i.e. $$w_n = 2$$)
1. if $$\lambda^1_1(n - 1) > \lambda^2_2(n - 1)$$ then
1. $$\lambda^2_2(n) = \lambda^2_2(n - 1) + 1$$
2. else (i.e. $$\lambda^1_1(n - 1) = \lambda^2_2(n - 1)$$)
1. $$\lambda^2_1(n) = \lambda^2_1(n - 1) + 1$$

Definition. We define binary operation $$\otimes$$ and $$\odot$$ on functions on $$\pint$$ as follows:

\begin{align} f \otimes g (n) = \min_{0 \le i \le n} \{f(i) + g(n) - g(i)\} \\ f \odot g (n) = \max_{0 \le i \le n} \{f(i) + g(n) - g(i)\} \end{align}

Claim. Given $$(\lambda^k_j(n))_{1 \le j \le k \le 2}$$ as the output Young tableau of column-inserting $$w_{1 : n}$$, we have

\begin{align} (\lambda^2_1, \lambda^2_2) = (S_2 \odot S_1, S_1 \otimes S_2). \end{align}

Proof. We use induction.

Basis. When $$n = 0$$, $$\lambda^2_1(n) = \lambda^2_2(n) = S_2 \odot S_1(n) = S_1 \otimes S_2(n) = 0$$.

Induction. Suppose $$(\lambda^2_1(n - 1), \lambda^2_2(n - 1)) = ((S_2 \odot S_1)(n - 1), (S_1 \otimes S_2)(n - 1))$$.

• When $$w_n = 1$$, i.e. $$S_1(n) - S_1(n - 1) = 1$$ and $$S_2(n) - S_2(n - 1) = 0$$.

Write $$M(l) = \max_{1 \le k \le l} \{S_2(k) - S_1(k)\}$$. Then $$M(n - 1) = M(n)$$ since $$S_1$$ increases at time $$n$$. Therefore by induction hypothesis (IH)

$(S_2 \odot S_1) (n) = \lambda^2_1 (n - 1) + 1 = \lambda^2_1(n); \qquad (S_1 \otimes S_2) (n) = \lambda^2_2(n - 1) = \lambda^2_2(n).$
• When $$w_n = 2$$, i.e. $$S_1(n) - S_1(n - 1) = 0$$ and $$S_2(n) - S_2(n - 1) = 1$$.

If $$\lambda^1_1(n - 1) > \lambda^2_2 (n - 1)$$, by IH, this means

$S_1(n - 1) > S_1(\ell) + S_2(n - 1) - S_2(\ell) \qquad (1)$

where $$\ell$$ is where $$S_2 - S_1$$ achieves maximum on $$1 : n - 1$$:

$M(n - 1) = S_2(\ell) - S_1(\ell).$

(1) gives

$S_2(n - 1) - S_1(n - 1) < S_2(\ell) - S_1(\ell)$

So

$S_2(n) - S_1(n) = S_2(n - 1) - S_1(n - 1) + 1 \le S_2(\ell) - S_1(\ell)$

i.e. $$\ell$$ is still the argmax. Hence

\begin{align} S_2 \odot S_1 (n) &= S_1(n) + M(n) = S_1(n - 1) + M(n - 1) = \lambda^2_1(n - 1) = \lambda^2_1 (n)\\ S_1 \otimes S_2 (n) &= S_2(n) - M(n) = 1 + S_2(n - 1) - M(n - 1) = \lambda^2_2(n - 1) + 1 = \lambda^2_2(n). \end{align}

If $$\lambda^1_1(n - 1) = \lambda^2_2 (n - 1)$$, then we have

$S_1(n - 1) = S_1(\ell) + S_2(n - 1) - S_2(\ell)$

i.e.

$S_2(n) - S_1(n) = S_2(n - 1) - S_1(n - 1) + 1 > S_2(\ell) - S_1(\ell).$

So

\begin{align} S_2 \odot S_1 (n) &= S_1(n) + M(n) = S_1(n - 1) + M(n - 1) + 1 = \lambda^2_1(n - 1) + 1 = \lambda^2_1(n)\\ S_1 \otimes S_2 (n) &= S_2(n) - M(n) = S_2(n - 1) + 1 - M(n - 1) - 1 = \lambda^2_2(n - 1) = \lambda^2_2(n). \end{align}

$$\square$$

## Rank-any

Now let us consider the case where the any positive integer $$k$$ may be inserted to the Young tableau, and observe what happens. As before we define path $$S_k$$ to be the number of $$k$$s so far:

$S_k(n) = \sum_{i \le n} 1_{w_i = k}. \qquad (1.3)$

When $$k$$ is inserted, first $$S_k$$ is increased by 1, and we try to insert $$k$$ from $$S_k$$ to the $$k$$th row.

1. If $$\lambda^{k - 1}_{k - 1}(n - 1) > \lambda^k_k (n - 1)$$ then we may insert $$k$$ to $$k$$th row:
1. $$\lambda^k_k (n) = \lambda^k_k (n - 1) + 1$$.
2. Now we try to insert $$k + 1$$ to the $$k$$th row:
1. If $$\lambda^k_{k - 1}(n - 1) > \lambda^{k + 1}_k (n - 1)$$ then we may insert $$k + 1$$ to $$k$$th row
1. $$\lambda^{k + 1}_k (n) = \lambda^{k + 1}_k(n - 1) + 1$$
2. ....
2. else (i.e. $$\lambda^{k - 1}_{k - 1}(n - 1) = \lambda^k_k (n - 1)$$) we can not insert $$k$$ from $$S_k$$ to the $$k$$th row
1. so we consider inserting $$k$$ to the $$k - 1$$th row
2. If $$\lambda^{k - 1}_{k - 2}(n - 1) > \lambda^k_{k - 1}(n - 1)$$ then we may insert $$k$$ to $$k - 1$$th row:
1. $$\lambda^k_{k - 1} (n) = \lambda^k_{k - 1} (n - 1) + 1$$
2. Now we try to insert $$k + 1$$ to the $$k - 1$$th row:
1. ...

One can see that the Rank-1 algorithm is iteratively applied here. So by the Claim, we can write

• $$\lambda^k_k = \lambda^{k - 1}_{k - 1} \otimes S_k$$
• $$\lambda^{k + 1}_k = \lambda^k_{k - 1} \otimes (S_{k + 1} \odot \lambda^k_k)$$
• ...
• $$\lambda^k_{k - 1} = \lambda^{k - 1}_{k - 2} \otimes (S_k \odot \lambda^{k - 1}_{k - 1})$$.
• ...

The Claim for general rank case is thus

Claim 1.

\begin{align} \lambda^1_1 &= S_1 \\ \lambda^k_k &= \lambda^{k - 1}_{k - 1} \otimes S_k, \qquad k \ge 2 \\ \mu^{k}_{k - 1} &:= S_k \odot \lambda^{k - 1}_{k - 1} = S_k + \lambda^{k - 1}_{k - 1} - \lambda^k_k, \qquad k \ge 2 \\ \lambda^k_1 &= \mu^{k}_1, \qquad k \ge 2 \\ \lambda^k_j &= \lambda^{k - 1}_{j - 1} \otimes \mu^{k}_{j}, \qquad 1 < j < k \\ \mu^{k}_{j - 1} &:= \mu^{k}_{j} \odot \lambda^{k - 1}_{j - 1} = \mu^{k}_{j} + \lambda^{k - 1}_{j - 1} - \lambda^k_j, \qquad 1 < j < k \\ \end{align}

We can write $$\mu^k_k := S_k$$ to obtain a simpler discription:

\begin{align} \mu^k_k &:= S_k \\ \lambda^k_1 &= \mu^k_1 \\ (\lambda^k_j, \mu^k_{j - 1}) &= (\lambda^{k - 1}_{j - 1} \otimes \mu^k_j, \mu^k_j \odot \lambda^{k - 1}_{j - 1}), \qquad 1 < j \le k \qquad (1.5) \end{align}

Corollary.

\begin{align} \lambda^k_k &= S_1 \otimes S_2 ... \otimes S_k \qquad (2) \\ \mu^k_j &= (\sum_{i = 1 : j} \lambda^k_i) - (\sum_{i = 1 : j - 1} \lambda^{k - 1}_i). \end{align}

This can be represented in nice pictures. If we use the following notation to represent the $$\odot$$ and $$\otimes$$ operations: $$\qquad \qquad \qquad (2.5)$$

Then the RS algorithm, say of rank 3 can be illustrated as follows: $$\qquad \qquad \qquad (3)$$

Claim 2. Let $$G_k$$ be defined as

\begin{align} G_1 (f) &= f \\ G_k (f_{1 : k}) &= (G_{k - 1} (f_2 \odot f_1, f_3 \odot (f_1 \otimes f_2), ..., f_k \odot (f_1 \otimes ... \otimes f_{k - 1})), f_1 \otimes ... \otimes f_k) \end{align}

Then

$\lambda^k_{1 : k} = G_k(S_{1 : k})$

Proof. Use induction.

Basis: $$G_1(S_1) = \lambda^1_1$$ is true because when the input is a string of ones the output is a one-row tableau looking the same as the input string.

Induction: Suppose $$\lambda^{k - 1}_{1 : k - 1} = G_{k - 1}(S_{1 : k - 1})$$. By (1.5) and (2) it suffices to show

$\lambda^k_{1 : k} = (G_{k - 1}(\mu^2_1, \mu^3_2, ..., \mu^k_{k - 1}), \lambda^k_k)$

namely

$G_{k - 1} (\mu^2_1, \mu^3_2, ..., \mu^k_{k - 1}) = \lambda^k_{1 : k - 1},$

which is true by looking at picture (3), removing the first row there, and using the IH. $$\square$$

## Pitman's transform

Definition. Given $$\ell$$ a positive integer, let $$e^i$$ for $$i = 1 : \ell$$ be the $$i$$th natural basis such that $$e^i_j = \delta_{ij}$$. let $$(P_i)_{i = 1 : \ell - 1}$$ be transformations of $$\ell$$-dimensional paths, defined as follows:

$(P_i f_{1 : \ell}) (n) = f(n) + \max_{0 \le k \le n} (f_{i + 1}(s) - f_i(s)) (e^i - e^{i + 1}) = (f_{1 : i - 1}, f_{i + 1} \odot f_i, f_i \otimes f_{i + 1}, f_{i + 2 : \ell})$

namely $$P_i$$ "performs" $$G_2$$ on the $$i$$th and $$i + 1$$th coordinates of its input. Let

$H_k = P_1 P_2 P_1 P_3 P_2 P_1 ... P_k P_{k - 1} ... P_1$

Claim 3. $$G_k = H_k$$.

Proof. Similar to the proof of Claim 2. We show that $$G_k$$ and $$H_k$$ define the same transformation by both agreeing to the RS algorithm:

$H_k S_{1 : k} = H_{k - 1} P_k P_{k - 1} ... P_1 S_{1 : k} = H_{k - 1} (\mu^2_1, \mu^3_2, \mu^4_3, ... ,\mu^k_{k - 1}, \lambda^k_k) = \lambda^k_{1 : k}$

by IH. $$\square$$

Remark. $$H_k$$ is the Pitman's transform of type A. It is claimed (see e.g. [{biane-bougerol-oconnell05}]) that the definition of $$H_k$$ does not depend on the order of the $$P$$s as long as their subscripts form a reduced decomposition of the longest element of reflection group $$A_{k - 1}$$:

$H_k = P_{i_1} P_{i_2} ... P_{i_m}$

whenever $$\sigma = s_{i_1} s_{i_2} ... s_{i_m}$$ where $$\sigma = (123...k)$$.

## Row insertion

For row insertion, one can follow similar derivations to obtain similar path transformations.

Rank-one.

\begin{align} \lambda^2_1 &= S_1 \odot S_2 \\ \lambda^2_2 &= S_2 \otimes S_1 \end{align}

Rank-any.

\begin{align} \mu^k_1 &= S_k \\ \lambda^k_k &= \mu^k_k \\ (\lambda^k_j, \mu^k_{j + 1}) &= (\lambda^{k - 1}_j \odot \mu^k_j, \mu^k_j \otimes \lambda^{k - 1}_j), \qquad j < k\\ \end{align}

And that

\begin{align} \lambda^k_1 &= S_1 \odot S_2 \odot ... \odot S_k \\ \mu^k_j &= (\sum_{i = 2 : k} \lambda^k_i) - (\sum_{i = 2 : k - 1} \lambda^{k - 1}_j). \end{align}

Illustrated, we have (note the cross arrows have the same meaning as in (2.5)) We can also define the Pitman's transform. For $$\ell$$-dimensional path $$f$$, let

$Q_i f := (f_{1 : i - 1}, f_{i + 1} \otimes f_i, f_i \odot f_{i + 1}, f_{i + 2 : \ell}).$

Then

Claim. Let $$w$$ be a word, and $$S$$ the corresponding walk / path as defined in (1.3). Let $$(\lambda^i_j)$$ be the output tableau obtained by row-inserting $$w$$, then for any $$k$$,

$(\lambda^k_k, \lambda^k_{k - 1}, ..., \lambda^k_1) = (Q_1 Q_2 Q_1 Q_3 Q_2 Q_1 ... Q_k Q_{k - 1} ... Q_1 S)_{1 : k}.$

## Pitman's transform taking non-word input

The definition of the Pitman's transform works for any paths, even those not corresponding to any words. Therefore it is more general than the RS algorithm. For example, it can take a matrix input like the RSK, by the matrix-to-path transform (1) in rsk_path_transformation. It can also naturally be defined on continuous-time paths by just subtituting $$\max$$ with $$\sup$$, see pitman_transform_type_a.

## References

• [biane-bougerol-oconnell05] Littelmann paths and Brownian paths, , Duke Mathematical Journal, Vol. 130, No. 1, p.127–167 2005.
• [oconnell03b] A path-transformation for random walks and the Robinson-Schensted correspondence, , Transactions of American Mathematical Society, Vol. 355, No. 9, p.3669–3697 2003.