# Discrete directed last passage percolations and directed polymers

directed_last_passage_percolation directed_polymer

Definition. The directed last passage percolation (DLPP) $$Z_0(n, m)$$ on integer lattice $$\pint_{>0} \times \pint_{>0}$$, where each vertex $$(i, j)$$ has weight $$a_{ij}$$, is defined as the maximum over directed paths from $$(1, 1)$$ to $$(n, m)$$:

$Z_0(n, m) = \max_{\pi: (1, 1) \to (n, m)} \sum_{(i, j) \in \pi_i} a_{ij}, \qquad (1)$

where recall from Greene's theorem that a directed path $$\pi: (1, 1) \to (n, m)$$ is a collection of coordinates $$(n_1, k_1) = (1, 1), (n_2, k_2), ..., (n_{r - 1}, k_{r - 1}), (n_r, k_r) = (n, m)$$ such that

$(n_i - n_{i - 1}, k_i - k_{i - 1}) \in \{(0, 1), (1, 0)\}.$

Definition. The directed polymer (DP) $$Z_1(n, m)$$ is defined as a geometric lifting of the $$Z_0(n, m)$$:

$Z_1(n, m) = \log \left(\sum_{\pi: (1, 1) \to (n, m)} \prod_{(i, j) \in \pi} e^{a_{ij}}\right). \qquad (2)$

The DLPP (or the Manhattan Tourist problem) is a typical example of dynamic programming, where one can rely on the recursive formula:

$Z_0(n, m) = (Z_0(n - 1, m) \vee Z_0(n, m - 1)) + a_{nm}. \qquad (3)$

Similarly for DP:

$Z_1(n, m) = \log(\exp(Z_1(n - 1, m)) + \exp(Z_1(n, m - 1))) + a_{nm}$

And these formulas in turn are alternative definitions of the DLPP and DP.

Let us focus on a square enclosed by vertices $$(n - 1, m - 1), (n - 1, m), (n, m - 1), (n, m)$$. By defining for $$b = 0, 1$$

\begin{align} U_b &= Z_b(n, m - 1) - Z_b(n - 1, m - 1) \\ V_b &= Z_b(n - 1, m) - Z_b(n - 1, m - 1) \\ X_b &= a_{n, m} \\ U_b' &= Z_b(n, m) - Z_b(n - 1, m) \\ V_b' &= Z_b(n, m) - Z_b(n, m - 1) \end{align}

By writing $$X_b' = U_b - U_b' + X_b$$ one can derive that $$U_b, V_b, X_b, U_b', V_b', X_b'$$ satisfy the Case 1 and 3 of the Burke property respectively when $$b = 0, 1$$.

Due to this link, we define the $$q$$- and $$qt$$-analog of the DLPP according to Case 4 and 5 of the Burke property:

Definition. We define the $$q$$- and $$qt$$-analog of the DLPP as follows:

\begin{align} Z (1, 1) &= a_{1, 1} \\ Z (n, 1) &= a_{n, 1} + Z (n - 1, 1), \qquad n > 1 \\ Z (1, m) &= a_{1, m} + Z (1, n - 1), \qquad m > 1 \\ Z (n, m) &= a_{n, m} + Z (n - 1, m) + Z (n, m - 1) - Z (n - 1, m - 1) - X'(n, m), \qquad m, n > 1. \end{align}

where

1. for $$q$$-analog: $X'(n, m) \sim q\text{Hyp}(Z(n, m - 1) - Z(n - 1, m - 1), \infty, Z(n - 1, m) - Z(n - 1, m - 1))$ where $$q$$Hyp is the q-hypergeometric distribution.
2. for $$qt$$-analog: $X'(n, m) \sim qt\text{IHyp}(Z(n, m - 1) - Z(n - 1, m - 1), Z(n - 1, m) - Z(n - 1, m - 1))$ where $$qt$$IHyp is the $$qt$$-infhypergeometric distribution defined in burke_property.

Remark. As per burke_property, the $$qt$$DLPP reduces to the $$q$$DLPP when $$t = 0$$, and to the usual DLPP when $$t = q$$.

Open. Find a global definition for the $$q$$- and $$qt$$-DLPP (like (1)(2) in the language of directed paths), i.e. analogues of greene_theorem restricted to the first row of the output tableaux.

The Burke property results in strong law of large numbers in the DLPP and DP models with equilibrium boundary conditions. Here we give the $$qt$$-version. The other cases are similar.

Definition. Let $$0 < \alpha, \beta < 1$$. The $$qt$$-deformed DLPP with equilibrium boundary condition is the one on integer lattice $$\pint \times \pint$$ with the following weights:

\begin{align} a_{0, 0} &\equiv 0 \\ a_{n, 0} &\sim qt\text{Geom}(\alpha), \qquad n > 0 \\ a_{0, m} &\sim qt\text{Geom}(\beta), \qquad m > 0 \\ a_{n, m} &\sim qt\text{Geom}(\alpha\beta), \qquad m, n > 0. \end{align}

Claim. Consider the $$qt$$-deformed DLPP with equilibrium boundary condition, we have almost surely

$\lim_{N \to \infty} {Z(\lfloor N x \rfloor, \lfloor N y \rfloor) \over N} = x \gamma(\alpha) + y \gamma(\beta),$

where $$\gamma(\alpha)$$ is the first moment of $$qt\text{Gemo}(\alpha)$$.

Proof. Similar to that of the version of DLPP with geometric weights ($$q = t$$) in e.g. Theorem 4.12 of [{romik14}]. $$\square$$

## References

• [romik14] The surprising mathematics of longest increasing subsequences, , 2014.