arXiv subjects

Sean D Lawley

Publications and source records attributed to Sean D Lawley.

At least 19 recordsLinked to original sources

arXiv2026

Search at bounded speed with immigration

Many biophysical search processes employ searchers which enter or "immigrate" into the domain progressively over time. Existing search time estimates can become unphysical for fast immigration since they imply that searchers move at infinite speed. In this paper, we investigate search times of immigrating searchers that move at bounded speed. In the fast immigration limit, we determine the full probability distribution and all the moments of the $k$th search time in terms of the early time probability distribution of a single searcher. We apply these rigorous mathematical results to several canonical models of stochastic search. We further analyze different models of "diffusion" and use these results to investigate when and how the minutiae of searcher dynamics affect search times. We compare our theory to numerical simulations.

math.PR↗

arXiv2026

Universality and ambiguity in extremes of anomalous diffusion

Many biophysical processes begin when the fastest searcher finds a target out of many random searchers, which is called an extreme or fastest first passage time (fFPT). In some models, (i) the fFPT vanishes logarithmically as the number of searchers grows, and (ii) the fFPT can be faster for subdiffusive search compared to normal diffusion. Though mathematically rigorous, the relevance of (i) and (ii) to actual physical systems is suspect since their derivations involve searchers which move with unbounded speed. Indeed, we previously proved that the fFPT for searchers with bounded speed converges exponentially to a strictly positive minimal search time as the number of searchers grows. In this paper, we study fFPTs for a broad class of anomalous and normal diffusion models with bounded or unbounded speed. These models include scaled Brownian motion, Riemann-Liouville fractional Brownian motion, and fractional Brownian motion. For all of these models, we show that the fFPT decays logarithmically in the number of searchers and that subdiffusion can be faster than normal diffusion (we further show that superdiffusion can be slower than normal diffusion). In this sense, features (i) and (ii) are rather universal. On the other hand, we show that the parameter regimes in which (i) and (ii) are valid depend on the particulars of the individual model, and thus ambiguities remain in the relevance of these features to specific physical systems.

cond-mat.stat-mech↗

arXiv2026

Channel transport: gating, geometry, and heterogeneous diffusion

Channel-mediated transport is ubiquitous in biology. A series of works by different theoreticians have sought to determine how the diffusive flux through a channel depends on (a) stochastic gating, (b) channel geometry, and (c) heterogeneous diffusion. In this paper, we derive an explicit estimate for the diffusive flux through a channel that accounts for these three factors. We show that our estimate is exact in certain parameter regimes. We further use stochastic simulations to confirm that our estimate remains accurate across a very broad range of parameters. Our estimate differs from some results in the physics literature.

cond-mat.stat-mech↗

arXiv2026

Diffusive flux into a stochastically gated tube

Diffusion-influenced reactions in the presence of gates which randomly open and close have been studied for decades in a variety of biophysical and biochemical scenarios. The diffusive flux from a large bulk reservoir to the end of a narrow tube with a stochastically gated entrance has been previously estimated. In this paper, we extend this gated flux estimate to be valid if (i) the tube is not necessarily narrow and/or (ii) the diffusivity differs in the tube versus the bulk. Extension (i) is challenging because it entails a nontrivial three-dimensional geometry. Extension (ii) is challenging because it introduces multiplicative noise. We derive an explicit flux estimate formula and prove that it is exact in certain parameter regimes. We further use stochastic simulations to show that the estimate remains accurate across a very broad range of parameters. Our results differ from prior work on extensions (i) and (ii).

cond-mat.stat-mech↗

arXiv2026

Yet another look at narrow escape through a tube

The narrow escape problem concerns the time needed for a diffusing particle to exit a confining domain through a small hole in the boundary. While this problem is now well-understood, determining the escape time for a particle that must exit through a narrow tube has proven challenging. Indeed, relying on analogies with electrodynamics, parameter fits to simulations, and heuristics, a variety of conflicting estimates for this escape time have been offered over the last three decades, some of which are counterintuitive and arguably non-physical. In this paper, we combine matched asymptotic analysis and probabilistic methods to determine the exact asymptotics of the narrow escape time through a tube. We obtain a new escape time formula which reduces to the various prior estimates in certain special cases. If the diffusivity in the tube differs from the diffusivity in the rest of the domain, our results reveal the importance of the form of the multiplicative noise inherent to any diffusivity that varies in space. We discuss our results in the context of asymmetric cell division.

cond-mat.stat-mech↗

arXiv2026

Stochastic search with space-dependent diffusivity

The canonical model of stochastic search tracks a randomly diffusing "searcher" until it finds a "target." Owing to its many applications across science and engineering, this perennially popular problem has been thoroughly investigated in a variety of models. However, aside from some exactly solvable one-dimensional examples, very little is known if the searcher diffusivity varies in space. For such space-dependent or "heterogeneous" diffusion, one must specify the interpretation of the multiplicative noise, which is termed the Itô-Stratonovich dilemma. In this paper, we investigate how stochastic search with space-dependent diffusivity depends on this interpretation. We obtain general formulas for the probability distribution and all the moments of the stochastic search time and the so-called splitting probabilities assuming that the targets are small or weakly reactive. These asymptotic results are valid for general space-dependent diffusivities in general domains in any space dimension with targets of general shape which may be in the interior or on the boundary of the domain. We illustrate our theory with stochastic simulations. Our analysis predicts that stochastic search can depend strongly and counterintuitively on the multiplicative noise interpretation.

cond-mat.stat-mech↗

arXiv2025

Escape from heterogeneous diffusion

Many physical processes depend on the time it takes a diffusing particle to find a target. Though this classical quantity is now well-understood in various scenarios, little is known if the diffusivity depends on the location of the particle. For such heterogeneous diffusion, an ambiguity arises in interpreting the stochastic process, which reflects the well-known Itô versus Stratonovich controversy. Here we analytically determine the mean escape time and splitting probabilities for an arbitrary heterogeneous diffusion in an arbitrary three-dimensional domain with small targets that can be perfectly or imperfectly absorbing. Our analysis reveals general principles for how search depends on heterogeneous diffusion and its interpretation (e.g. Itô, Stratonovich, or kinetic). An intricate picture emerges in which, for instance, increasing the diffusivity can decrease, not affect, or even increase the escape time. Our results could be used to determine the appropriate interpretation for specific physical systems.

cond-mat.stat-mech↗

arXiv2025

Passage times of fast inhomogeneous immigration processes

In many biophysical systems, key events are triggered when the fastest of many random searchers find a target. Most mathematical models of such systems assume that all searchers are initially present in the search domain, which permits the use of classical extreme value theory. In this paper, we explore $k$th passage times of inhomogeneous immigration processes where searchers are added to the domain over time either through time inhomogeneous rates or a Yule (pure birth) process. We rigorously prove convergence in distribution and convergence of moments of the $k$th passage times for both processes as immigration rates grow. In particular, we relate immigration with time inhomogeneous rates to previous work where all searchers are initially present through a coupling argument and demonstrate how immigration through a Yule process can be viewed as a time inhomogeneous immigration process with a random time shift. For Yule immigration, we find that the extreme distributions depart from the classical family of Frechet, Gumbel, and Weibull, and we compare our results to classical theorems on branching Brownian motion. This work offers one of the few examples where extreme value distributions can be obtained exactly for random variables which are neither independent nor identically distributed.

math.PR↗

arXiv2025

Cover times with stochastic resetting

Cover times quantify the speed of exhaustive search. In this work, we compute exactly the mean cover time associated with a one-dimensional Brownian search under exponentially distributed resetting. We also approximate the moments of cover times of a wide range of stochastic search processes in $d$-dimensional continuous space and on an arbitrary discrete network under frequent stochastic resetting. These results hold for a large class of resetting time distributions and search processes including diffusion and Markov jump processes.

math.PR↗

arXiv2025

First passage times with fast immigration

Many scientific questions can be framed as asking for a first passage time (FPT), which generically describes the time it takes a random "searcher" to find a "target." The important timescale in a variety of biophysical systems is the time it takes the fastest searcher(s) to find a target out of many searchers. Previous work on such fastest FPTs assumes that all searchers are initially present in the domain, which makes the problem amenable to extreme value theory. In this paper, we consider an alternative model in which searchers progressively enter the domain at a constant "immigration" rate. In the fast immigration rate limit, we determine the probability distribution and moments of the $k$-th fastest FPT. Our rigorous theory applies to many models of stochastic motion, including random walks on discrete networks and diffusion on continuous state spaces. Mathematically, our analysis involves studying the extrema of an infinite sequence of random variables which are both not independent and not identically distributed. Our results constitute a rare instance in which extreme value statistics can be determined exactly for strongly correlated random variables.

math.PR↗

arXiv2025

Reaction kinetics of membrane receptors: a spatial modeling approach

The interactions between diffusing molecules and membrane-bound receptors drive numerous cellular processes. In this work, we develop a spatial model of molecular interactions with membrane receptors by homogenizing the cell membrane and describing the evolution of both molecular diffusion and molecule-receptor interactions. By analyzing a resulting partial differential equation coupled to ordinary differential equations, we derive analytical expressions for the steady-state molecular influx rate in four prototypical interaction scenarios: Michaelis-Menten kinetics, Substrate Competition, Competitive Inhibition, and Uncompetitive Inhibition. For each scenario, we show how to modify the classical well-mixed reaction rate theory to resolve spatial features inherent to receptors bound to cell membranes. We find that naive well-mixed calculations significantly overestimate reaction rates in certain biophysical parameter regimes.

q-bio.QM↗

arXiv2024

Cover times of many diffusive or subdiffusive searchers

Cover times measure the speed of exhaustive searches which require the exploration of an entire spatial region(s). Applications include the immune system hunting pathogens, animals collecting food, robotic demining or cleaning, and computer search algorithms. Mathematically, a cover time is the first time a random searcher(s) comes within a specified "detection radius" of every point in the target region (often the entire spatial domain). Due to their many applications and their fundamental probabilistic importance, cover times have been extensively studied in the physics and probability literatures. This prior work has generally studied cover times of a single searcher with a vanishing detection radius or a large target region. This prior work has further claimed that cover times for multiple searchers can be estimated by a simple rescaling of the cover time of a single searcher. In this paper, we study cover times of many diffusive or subdiffusive searchers and show that prior estimates break down as the number of searchers grows. We prove a rather universal formula for all the moments of such cover times in the many searcher limit that depends only on (i) the searcher's characteristic (sub)diffusivity and (ii) a certain geodesic distance between the searcher starting location(s) and the farthest point in the target. This formula is otherwise independent of the detection radius, space dimension, target size, and domain size. We illustrate our results in several examples and compare them to detailed stochastic simulations.

math.PR↗

arXiv2024

Cover times of many random walkers on a discrete network

The speed of an exhaustive search can be measured by a cover time, which is defined as the time it takes a random searcher to visit every state in some target set. Cover times have been studied in both the physics and probability literatures, with most prior works focusing on a single searcher. In this paper, we prove an explicit formula for all the moments of the cover time for many searchers on an arbitrary discrete network. Our results show that such cover times depend only on properties of the network along the shortest paths to the most distant parts of the target. This mere local dependence contrasts with the well-known result that cover times for single searchers depend on global properties of the network. We illustrate our rigorous results by stochastic simulations.

cond-mat.stat-mech↗

arXiv2024

Boundary homogenization for partially reactive patches

A wide variety of physical, chemical, and biological processes involve diffusive particles interacting with surfaces containing reactive patches. The theory of boundary homogenization seeks to encapsulate the effective reactivity of such a patchy surface by a single trapping rate parameter. In this paper, we derive the trapping rate for partially reactive patches occupying a small fraction of a surface. We use matched asymptotic analysis, double perturbation expansions, and homogenization theory to derive formulas for the trapping rate in terms of the far-field behavior of solutions to certain partial differential equations (PDEs). We then develop kinetic Monte Carlo (KMC) algorithms to rapidly compute these far-field behaviors. These KMC algorithms depend on probabilistic representations of PDE solutions, including using the theory of Brownian local time. We confirm our results by comparing to KMC simulations of the full stochastic system. We further compare our results to prior heuristic approximations.

cond-mat.stat-mech↗

arXiv2023

Competition of many searchers

First passage times (FPTs) are often used to study timescales in physical, chemical, and biological processes. FPTs generically describe the time it takes a random "searcher" to find a "target." In many systems, the important timescale is not the time it takes a single searcher to find a target, but rather the time it takes the fastest searcher out of many searchers to find a target. Such fastest FPTs or extreme FPTs result from many searchers competing to find the target and differ markedly from FPTs of single searchers. In this chapter, we review recent results on fastest FPTs. We show how fastest FPTs depend on the mode of stochastic search (including search by diffusion, subdiffusion, superdiffusion, and discrete jumps), the initial searcher distribution, and properties of the spatial domain.

cond-mat.stat-mech↗

arXiv2023

First Hitting Time of a One-Dimensional Levy Flight to Small Targets

First hitting times (FHTs) describe the time it takes a random "searcher" to find a "target" and are used to study timescales in many applications. FHTs have been well-studied for diffusive search, especially for small targets, which is called the narrow capture or narrow escape problem. In this paper, we study the first hitting time to small targets for a one-dimensional superdiffusive search described by a Levy flight. By applying the method of matched asymptotic expansions to a fractional differential equation we obtain an explicit asymptotic expansion for the mean FHT (MFHT). For fractional order $s\in(0,1)$ (describing a $(2s)$-stable Levy flight whose squared displacement scales as $t^{1/s}$ in time $t$) and targets of radius $\varepsilon\ll1$, we show that the MFHT is order one for $s\in(1/2,1)$ and diverges as $\log(1/\varepsilon)$ for $s=1/2$ and $\varepsilon^{2s-1}$ for $s\in(0,1/2)$. We then use our asymptotic results to identify the value of $s\in(0,1]$ which minimizes the average MFHT and find that (a) this optimal value of $s$ vanishes for sparse targets and (b) the value $s=1/2$ (corresponding to an inverse square Levy search) is optimal in only very specific circumstances. We confirm our results by comparison to both deterministic numerical solutions of the associated fractional differential equation and stochastic simulations.

cond-mat.stat-mech↗

arXiv2023

Slowest first passage times, redundancy, and menopause timing

Biological events are often initiated when a random "searcher" finds a "target," which is called a first passage time (FPT). In some biological systems involving multiple searchers, an important timescale is the time it takes the slowest searcher(s) to find a target. For example, of the hundreds of thousands of primordial follicles in a woman's ovarian reserve, it is the slowest to leave that trigger the onset of menopause. Such slowest FPTs may also contribute to the reliability of cell signaling pathways and influence the ability of a cell to locate an external stimulus. In this paper, we use extreme value theory and asymptotic analysis to obtain rigorous approximations to the full probability distribution and moments of slowest FPTs. Though the results are proven in the limit of many searchers, numerical simulations reveal that the approximations are accurate for any number of searchers in typical scenarios of interest. We apply these general mathematical results to models of ovarian aging and menopause timing, which reveals the role of slowest FPTs for understanding redundancy in biological systems. We also apply the theory to several popular models of stochastic search, including search by diffusive, subdiffusive, and mortal searchers.

physics.bio-ph↗

arXiv2023

First passage times under frequent stochastic resetting

We determine the full distribution and moments of the first passage time for a wide class of stochastic search processes in the limit of frequent stochastic resetting. Our results apply to any system whose short-time behavior of the search process without resetting can be estimated. In addition to the typical case of exponentially distributed resetting times, we prove our results for a wide array of resetting time distributions. We illustrate our results in several examples and show that the errors of our approximations vanish exponentially fast in typical scenarios of diffusive search.

cond-mat.stat-mech↗