Oscillation of partition functions under very strong disorder
Let $d \ge 2$, $\mathcal R = \{e_1, \dots, e_d\}$, and let $(X_n)_{n \ge 0}$ be the random walk on $\mathbb Z^d$ from $0$ with independent steps uniform on $\mathcal R$; $E_0$ denotes expectation over the walk. The environment $\omega = (\omega_x)_{x \in \mathbb Z^d}$ is i.i.d. with values in a Borel set $\Gamma \subseteq \mathbb R$, independent of the walk, and $(T_x\omega)_y = \omega_{x+y}$. The potential is local, $V(\omega, z) = V_o(\omega_0, z)$ for a measurable $V_o : \Gamma \times \mathcal R \to \mathbb R$, and may depend on the step $z$. Assume $V \in \mathcal L$: for all $z, z' \in \mathcal R$, $\mathbb E|V(\omega, z)| < \infty$ and almost surely $$\limsup_{\delta \downarrow 0} \limsup_{n \to \infty} \max_{x \in D_1 \cup \dots \cup D_n} \frac1n \sum_{0 \le i \le \delta n} |V(T_{x + iz'}\omega, z)| = 0,$$ where $D_j = \{x \in \mathbb Z_+^d : |x|_1 = j\}$; a finite $p$-th moment of every $V(\cdot, z)$ for some $p > d$ suffices. Let $$Z_n^\omega = E_0 \exp\Bigl\{ \sum_{i=0}^{n-1} V(T_{X_i}\omega, X_{i+1} - X_i) \Bigr\}, \qquad \Lambda_a = \log\Bigl( \frac1d \sum_{z \in \mathcal R} \mathbb E\, e^{V(\omega, z)} \Bigr) \in (-\infty, \infty].$$ The quenched free energy $\Lambda_q = \lim_{n \to \infty} \frac1n \log Z_n^\omega$ exists almost surely and is deterministic, and $\Lambda_q \le \Lambda_a$; very strong disorder means $\Lambda_q < \Lambda_a$. Rassoul-Agha, Seppäläinen and Yilmaz showed that, when $\Lambda_q < \infty$, each of the events $\{\liminf_n e^{-n\Lambda_q} Z_n^\omega = 0\}$ and $\{\limsup_n e^{-n\Lambda_q} Z_n^\omega = \infty\}$ has probability $0$ or $1$, and, using a theorem of Borodin, Corwin and Remenik, that the $\limsup$ is almost surely infinite for the log-gamma directed polymer on $\mathbb Z^2$ with a small enough parameter, which is in very strong disorder.
Problem. Assume $\Lambda_q < \infty$ and very strong disorder, $\Lambda_q < \Lambda_a \le \infty$. Is it always true that, almost surely, $$\liminf_{n \to \infty} e^{-n\Lambda_q} Z_n^\omega = 0 \quad \text{and} \quad \limsup_{n \to \infty} e^{-n\Lambda_q} Z_n^\omega = \infty \, ?$$
Source: Rassoul-Agha, Seppäläinen and Yilmaz (2017), Conjecture 2.13.
Imported from github.com/MColbrook/AIM.
Classification
- Primary
- PR — Probability
- Secondary
- MP
- Keywords
- AIM Open Applied Problems, directed polymers, random walks in random potentials
References
@article{rassoulagha2017,
author = {Rassoul-Agha, Firas and Sepp{\"a}l{\"a}inen, Timo and Yilmaz, Atilla},
title = {Variational formulas and disorder regimes of random walks in random potentials},
journal = {Bernoulli},
volume = {23},
number = {1},
year = {2017},
pages = {405--431},
doi = {10.3150/15-BEJ747},
eprint = {1410.4474},
archivePrefix = {arXiv},
}
@article{borodin2013,
author = {Borodin, Alexei and Corwin, Ivan and Remenik, Daniel},
title = {Log-gamma polymer free energy fluctuations via a {F}redholm determinant identity},
journal = {Comm. Math. Phys.},
volume = {324},
number = {1},
year = {2013},
pages = {215--232},
doi = {10.1007/s00220-013-1750-x},
eprint = {1206.4573},
archivePrefix = {arXiv},
}
@misc{aim-polymer-oscillation,
author = {Brunton, Steve and Colbrook, Matthew J. and de Hoop, Maarten V. and Stepaniants, George and Townsend, Alex and Ward, Rachel},
title = {{Oscillation of partition functions under very strong disorder}},
howpublished = {AIM --- Open Applied Problems, GitHub repository},
year = {2026},
url = {https://github.com/MColbrook/AIM/blob/a153f469ee0ea64e876b2ad97c79d252c19a3e22/problems/642-polymer-oscillation.md},
note = {Entry 642 at commit a153f46, 2026-10-13},
}
Citing this problem
This identifier is permanent. It will not change if the problem is solved, reclassified or withdrawn.
OP-00298, "Oscillation of partition functions under very strong disorder", clopen.solutions.