arXiv subjects

F. den Hollander

Publications and source records attributed to F. den Hollander.

23 records · Page 2Linked to original sources

arXiv2007

Intermittency in a catalytic random medium

In this paper, we study intermittency for the parabolic Anderson equation $\partial u/\partial t=κΔu+ξu$, where $u:\mathbb{Z}^d\times [0,\infty)\to\mathbb{R}$, $κ$ is the diffusion constant, $Δ$ is the discrete Laplacian and $ξ:\mathbb{Z}^d\times[0,\infty)\to\mathbb {R}$ is a space-time random medium. We focus on the case where $ξ$ is $γ$ times the random medium that is obtained by running independent simple random walks with diffusion constant $ρ$ starting from a Poisson random field with intensity $ν$. Throughout the paper, we assume that $κ,γ,ρ,ν\in (0,\infty)$. The solution of the equation describes the evolution of a ``reactant'' $u$ under the influence of a ``catalyst'' $ξ$. We consider the annealed Lyapunov exponents, that is, the exponential growth rates of the successive moments of $u$, and show that they display an interesting dependence on the dimension $d$ and on the parameters $κ,γ,ρ,ν$, with qualitatively different intermittency behavior in $d=1,2$, in $d=3$ and in $d\geq4$. Special attention is given to the asymptotics of these Lyapunov exponents for $κ\downarrow0$ and $κ\to\infty$.

math.PR↗

arXiv2006

Intermittency on catalysts: symmetric exclusion

We continue our study of intermittency for the parabolic Anderson equation $\partial u/\partial t = κΔu + ξu$, where $u\colon \Z^d\times [0,\infty)\to\R$, $κ$ is the diffusion constant, $Δ$ is the discrete Laplacian, and $ξ\colon \Z^d\times [0,\infty)\to\R$ is a space-time random medium. The solution of the equation describes the evolution of a ``reactant'' $u$ under the influence of a ``catalyst'' $ξ$. In this paper we focus on the case where $ξ$ is exclusion with a symmetric random walk transition kernel, starting from equilibrium with density $ρ\in (0,1)$. We consider the annealed Lyapunov exponents, i.e., the exponential growth rates of the successive moments of $u$. We show that these exponents are trivial when the random walk is recurrent, but display an interesting dependence on the diffusion constant $κ$ when the random walk is transient, with qualitatively different behavior in different dimensions. Special attention is given to the asymptotics of the exponents for $κ\to\infty$, which is controlled by moderate deviations of $ξ$ requiring a delicate expansion argument. In Gärtner and den Hollander \cite{garhol04} the case where $ξ$ is a Poisson field of independent (simple) random walks was studied. The two cases show interesting differences and similarities. Throughout the paper, a comparison of the two cases plays a crucial role.

math.PR↗

arXiv2002

Weak-interaction limits for one-dimensional random polymers

In this paper we present a new and flexible method to show that, in one dimension, various self-repellent random walks converge to self-repellent Brownian motion in the limit of weak interaction after appropriate space-time scaling. Our method is based on cutting the path into pieces of an appropriately scaled length, controlling the interaction between the different pieces, and applying an invariance principle to the single pieces. In this way we show that the self-repellent random walk large deviation rate function for the empirical drift of the path converges to the self-repellent Brownian motion large deviation rate function after appropriate scaling with the interaction parameters. The method is considerably simpler than the approach followed in our earlier work, which was based on functional analytic arguments applied to variational representations and only worked in a very limited number of situations. We consider two examples of a weak interaction limit: (1) vanishing self-repellence, (2) diverging step variance. In example (1), we recover our earlier scaling results for simple random walk with vanishing self-repellence and show how these can be extended to random walk with steps that have zero mean and a finite exponential moment. Moreover, we show that these scaling results are stable against adding self-attraction, provided the self-repellence dominates. In example (2), we prove a conjecture by Aldous for the scaling of self-avoiding walk with diverging step variance. Moreover, we consider self-avoiding walk on a two-dimensional horizontal strip such that the steps in the vertical direction are uniform over the width of the strip and find the scaling as the width tends to infinity.

math.PR↗

arXiv2002

Large deviations for the one-dimensional Edwards model

In this paper we prove a large deviation principle for the empirical drift of a one-dimensional Brownian motion with self-repellence called the Edwards model. Our results extend earlier work in which a law of large numbers, respectively, a central limit theorem were derived. In the Edwards model a path of length $T$ receives a penalty $e^{-βH_T}$, where $ H_T$ is the self-intersection local time of the path and $β\in(0,\infty)$ is a parameter called the strength of self-repellence. We identify the rate function in the large deviation principle for the endpoint of the path as $β^{\frac 23} I(β^{-\frac 13}\cdot)$, with $I(\cdot)$ given in terms of the principal eigenvalues of a one-parameter family of Sturm-Liouville operators. We show that there exist numbers $0<b^{**}<b^*<\infty$ such that: (1) $I$ is linearly decreasing on $[0,b^{**}]$; (2) $I$ is real-analytic and strictly convex on $(b^{**},\infty)$; (3) $I$ is continuously differentiable at $b^{**}$; (4) $I$ has a unique zero at $b^*$. (The latter fact identifies $b^*$ as the asymptotic drift of the endpoint.) The critical drift $b^{**}$ is associated with a crossover in the optimal strategy of the path: for $b\geq b^{**}$ the path assumes local drift $b$ during the full time $T$, while for $0\leq b<b^{**}$ it assumes local drift $b^{**}$ during time $\frac{b^{**}+b}{2b^{**}}T$ and local drift $-b^{**}$ during the remaining time $\frac{b^{**}-b}{2b^{**}}T$. Thus, in the second regime the path makes an overshoot of size $\frac{b^{**}-b}{2}T$ in order to reduce its intersection local time.

math.PR↗

arXiv2001

Possible loss and recovery of Gibbsianness during the stochastic evolution of Gibbs measures

We consider Ising-spin systems starting from an initial Gibbs measure $ν$ and evolving under a spin-flip dynamics towards a reversible Gibbs measure $μ\not=ν$. Both $ν$ and $μ$ are assumed to have a finite-range interaction. We study the Gibbsian character of the measure $νS(t)$ at time $t$ and show the following: (1) For all $ν$ and $μ$, $νS(t)$ is Gibbs for small $t$. (2) If both $ν$ and $μ$ have a high or infinite temperature, then $νS(t)$ is Gibbs for all $t>0$. (3) If $ν$ has a low non-zero temperature and a zero magnetic field and $μ$ has a high or infinite temperature, then $νS(t)$ is Gibbs for small $t$ and non-Gibbs for large $t$. (4) If $ν$ has a low non-zero temperature and a non-zero magnetic field and $μ$ has a high or infinite temperature, then $νS(t)$ is Gibbs for small $t$, non-Gibbs for intermediate $t$, and Gibbs for large $t$. The regime where $μ$ has a low or zero temperature and $t$ is not small remains open. This regime presumably allows for many different scenarios.

math-ph↗