# Dynamics of type-A Pitman's transform

pitman_transform robinson_schensted local_time robinson_schensted_knuth

In this article we derive the dynamical equations for the Pitman's transform of type A. This derivation is not very useful due to the "niceness" assumption. It is more of a proof of concept. Related readings include [{warren07}] and [{oconnell12a}].

For paths $$A$$, let $$L_A$$ be the time $$A$$ spends at $$0$$:

$L_A(t) = \int_0^t 1_{A(s) = 0} ds.$

It is similar to local time, except the latter is the time $$A$$ spends near some point ($$0$$ here). Also we only consider deterministic paths in this article, as opposed to local time which is usually considered with respect to stochastic processes. For $$A$$ with good regularities, $$L_A$$ and the local time should be the same.

Claim. Given paths $$S_1, S_2, ...$$ that are "nice" enough, and let $$(\lambda^k_j)_{1 \le j \le k}$$ be the output after applying Pitman's transform (column insertion) to the $$S$$'s. Then

\begin{align} d \lambda^1_1 &= d S_1 \\ d \lambda^k_1 &= d \lambda^{k - 1}_1 + d L_{\lambda^{k - 1}_1 - \lambda^k_2}, \qquad k > 1\\ d \lambda^k_k &= d S_k - d L_{\lambda^{k - 1}_{k - 1} - \lambda^k_k}, \qquad k > 1 \\ d \lambda^k_j &= d \lambda^{k - 1}_j + d L_{\lambda^{k - 1}_j - \lambda^k_{j + 1}} - d L_{\lambda^{k - 1}_{j - 1} - \lambda^k_j} \qquad 1 < j < k \end{align}

To prove the Claim, we use a lemma.

Lemma. For paths $$A, B$$ that are "nice" enough we have

\begin{align} d A \otimes B &= d B - d L_{A \otimes B - A} \\ d A \odot B &= d B + d L_{A - A \odot B} \end{align}

Proof. We show the formula for $$A \otimes B$$, as $$A \odot B$$ is similar. Recall

$A \otimes B (t) = B(t) - \sup_{0 \le s \le t} \{B(s) - A(s)\}$

and hence

$(A \otimes B - A) (t) = (B - A)(t) - \sup_{0 \le s \le t} \{B(s) - A(s)\}$

so it suffices to show that

$d M(t) = d L_{M - C}(t) \qquad (1)$

where $$C = B - A$$ and $$M(t) = \sup_{0 \le s \le t} C(s)$$.

Now we use the assumption of "nice". Suppose there does not exist an interval on which $$C$$ is constant. Then we know that $$M$$ increases if and only if $$C$$ reaches $$M$$, which implies (1) and we are done. $$\square$$

Proof of Claim. Recall from pitman_transform_type_a and the Corollary from rs_pitman_type_a that

\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 j > 1. \\ \mu^k_j &= (\sum_{i = 1 : j} \lambda^k_i) - (\sum_{i = 1 : j - 1} \lambda^{k - 1}_i). \end{align}

To derive equations for $$\lambda^k_j$$ we consider the four cases where $$j = k = 1$$, $$j = 1 < k$$, $$j = k > 1$$ or $$1 < j < k$$. We use the Lemma repeatedly.

1. $$j = k = 1$$. $d \lambda^1_1 = d \mu^1_1 = d S_1$
2. $$j = 1 < k$$. $d \lambda^k_1 = d \mu^k_1 = d \mu^k_2 \odot \lambda^{k - 1}_1 = d \lambda^{k - 1}_1 - d L_{\mu^k_2 - \lambda^k_1} = d \lambda^{k - 1}_1 - d L_{\lambda^k_2 - \lambda^{k - 1}_1}$
3. $$j = k > 1$$. $d \lambda^k_k = d \lambda^{k - 1}_{k - 1} \otimes \mu^k_k = d \mu^k_k - d L_{\lambda^k_k - \lambda^{k - 1}_{k - 1}} = d S_k - d L_{\lambda^k_k - \lambda^{k - 1}_{k - 1}}$
4. $$1 < j < k$$. $d \lambda^k_j = d \lambda^{k - 1}_{j - 1} \otimes \mu^k_j = d \mu^k_j - d L_{\lambda^k_j - \lambda^{k - 1}_{j - 1}}$ where we then use $d \mu^k_j = d \mu^k_{j + 1} \odot \lambda^{k - 1}_j = d \lambda^{k - 1}_j + d L_{\mu^k_{j + 1} - \mu^k_j}$ where we then use $\mu^k_{j + 1} - \mu^k_j = \lambda^k_{j + 1} - \lambda^{k - 1}_j$

$$\square$$

The equation for the row insertion transform is similar:

\begin{align} d \lambda^1_1 &= d S_1 \\ d \lambda^k_1 &= d S_k + d L_{\lambda^{k - 1}_1 - \lambda^k_1}, \qquad k > 1 \\ d \lambda^k_k &= d \lambda^{k - 1}_{k - 1} - d L_{\lambda^{k - 1}_{k - 1} - \lambda^k_{k - 1}}, \qquad k > 1 \\ d \lambda^k_j &= d \lambda^{k - 1}_{j - 1} - d L_{\lambda^{k - 1}_{j - 1} - \lambda^k_{j - 1}} + d L_{\lambda^{k - 1}_j - \lambda^k_j}. \qquad 1 < j < k \end{align}

Remark. Again, by using the same involution as in Pitman's transform of type A, the symmetry between row and column insertion is crystal clear.

## References

• [oconnell12a] Directed Polymers and the Quantum Toda Lattice, , The Annals of Probability, Vol. 40, No. 2, p.437–458 2012.
• [warren07] Dyson's Brownian motions, intertwining and interlacing, , Electronic Journal of Probability, Vol. 12, p.573–590 2007.