arXiv subjects

Wenjie Ni

Publications and source records attributed to Wenjie Ni.

13 recordsLinked to original sources

arXiv2026

The fractional Fisher-KPP equation with free boundaries

In this paper, we consider a free boundary problem with fractional diffusion $(-Δ)^s$ $(0<s<1)$, as a model for species spreading, which can be viewed as a natural extension of the free boundary model in \cite{CDLL2019}, where the nonlocal diffusion term is defined via a continuous integrable kernel function $J(x)$. This fractional diffusion model can also be viewed as a nonlocal version of the free boundary model in \cite{DuLin2010}, whose local diffusion term is given by the classical Laplacian. We first establish the global existence and uniqueness of the solution, which is the main contribution of this paper. This well-posedness question has been open for some time now due to the difficulties caused by the singularity and non-integrability of the associated kernel function of $(-Δ)^s$, and the lack of general enough regularity results for operators extending the standard fractional Laplacian. A crucial step that enables us to answer this question relies on an approximation approach, which breaks an associated linear fractional parabolic initial boundary value problem with curved boundaries into a sequence of approximating problems, where the functions representing the curved boundaries are replaced by approximating step functions. We then show that, for Fisher-KPP type nonlinear growth terms, the long-time dynamics of the model exhibits a spreading-vanishing dichotomy, similar to the earlier models of \cite{DuLin2010, CDLL2019}. More precise descriptions of the spreading profile of the solution will be considered in a separate work.

math.AP↗

arXiv2026

Trichotomy dynamics of a free boundary model for biological invasion

It is well known that the reaction-diffusion equation $u_t=du_{xx}+f(u)$ with compactly supported nonnegative initial functions exhibits trichotomy dynamics for bistable and combustion type $f(u)$ \cite{DM, zlatos}. The same is true for the corresponding Stefan type free boundary problem \cite{DL}. In this paper, we reveal a rather different type of trichotomy for this reaction-diffusion equation under a new set of (free) boundary conditions, arising as a model for biological invasion with $u(t,x)$ representing the density of an invading species over the one dimensional spatial regin $[0, h(t)]$. The evolution of the invading front $x=h(t)$ is governed by $h'(t)=-\frac dδu_x(t, h(t))$ and $u(t, h(t))=δ\in (\hatθ_f, 1)$, with $\hatθ_f \in [0, 1)$ uniquely determined by $f$; they allow $h(t)$ to advance as well as to retreat when time increases. At the fixed boundary $x=0$, the density is controlled by $u(t,0)=δ_0\geq 0$. We completely classify the long-time dynamics of the model when $f(u)$ is a monostable, or bistable, or combustion type nonlinear function. In the biologically interesting case that $δ_0<δ$, we show that there are exactly three scenarios: (i) successful spreading, (ii) finite-time vanishing, (iii) a transition state characterized by $h(t)\to l_*\in (0, \infty)$ and $u(t,x)\to w_*(x)$ as $t\to\infty$, where $(u(t,x), h(t))\equiv (w_*(x), l_*)$ is the unique stationary solution of the free boundary problem. The model here does not have the usual order-preserving property enjoyed by those considered in \cite{DM, zlatos, DL} and elsewhere (i.e., $u(0,x)\leq v(0,x)$ implies $u(t,x)\leq v(t,x)$ for all $t>0$ if $u$ and $v$ are two solutions of the problem), which is intrinsically linked to the many novel features of the model.

math.AP↗

arXiv2025

Convergence to a receding wave in a monostable free boundary problem

We study a monostable reaction-diffusion equation of the form $u_t=du_{xx}+f(u)$ over a semi-infinite spatial domain $[g(t),\infty)$, with $x=g(t)$ the free boundary whose evolution is governed by equations derived from a ``preferred population density'' principle, which postulates that the species with population density $u(t,x)$ and population range $[g(t),\infty)$ maintains a certain density $δ$ at the habitat edge $x=g(t)$. In the ``high-density'' regime, where $δ$ exceeds the carrying capacity of the favourable environment represented by a monostable function $f(u)$, it is known (see \cite{DLNS} for the case of a bounded population range $[g(t), h(t)]$) that for large time, the front retreats as time advances. In this work, the unboundedness of the population range $[g(t),\infty)$ allows us to prove that, as time $t$ converges to infinity, the free boundary $x=g(t)$ converges to $\infty$ with a constant asymptotic speed $c(δ)>0$ determined by an associated semi-wave problem, and the population density $u(t,x)$ has the property that $u(t,x+g(t))$ converges uniformly to $q_{c(δ)}(x)$, the semi-wave profile function associated with the speed $c(δ)$. It turns out that in the retreating situation considered here, some key techniques developed for advancing fronts in related free boundary models do not work anymore. This difficulty is overcome here by a ``touching method", which uses a family of lower and upper solutions constructed from semi-waves of some carefully designed auxiliary problems to touch the solution $u(t,x)$ at the moving boundary $x=g(t)$, thereby generating a setting where the comparison principle can be used to obtain the desired estimates for $g'(t)$ and $u(t,x)$. We believe this method will find applications elsewhere.

math.AP↗

arXiv2025

Asymptotic limit of the principal eigenvalue of asymmetric nonlocal diffusion operators and propagation dynamics

For fixed $c\in\mathbb R$, $l>0$ and a general non-symmetric kernel function $J(x)$ satisfying a standard assumption, we consider the nonlocal diffusion operator \begin{align*} \bf{L}^{J, c}_{(-l,l)}[ϕ](x):=\int_{-l}^lJ(x-y)ϕ(y)\,dy+cϕ'(x), \end{align*} and prove that its principal eigenvalue $λ_p(\bf{L}^{J, c}_{(-l,l)})$ has the following asymptotic limit: \begin{equation*}\label{l-to-infty-c} \lim\limits_{l\to \infty}λ_p(\bf {L}^{J, c}_{(-l,l)})=\inf\limits_{ν\in\mathbb{R}}\big[\int_{\mathbb{R}}J(x)e^{-νx}\,dx+cν\big]. \end{equation*} We then demonstrate how this result can be applied to determine the propagation dynamics of the associated Cauchy problem \begin{equation*} \label{cau} \left\{ \begin{array}{ll} \displaystyle u_t = d \big[\int_{\mathbb{R}} J(x-y) u(t,y) \, dy - u(t,x)\big] + f(u), & t > 0, \; x \in \mathbb{R}, u(0, x) = u_0(x), & x \in \mathbb{R}, \end{array} \right. \end{equation*} with a KPP nonlinear term $f(u)$. This provides a new approach to understand the propagation dynamics of KPP type models, very different from those based on traveling wave solutions or on the dynamical systems method of Weinberger (1982).

math.AP↗

arXiv2025

Invasion dynamics of super invaders: Elimination of Allee effects by a strategy at the range boundary

Using a reaction-diffusion model with free boundaries in one space dimension for a single population species with density $u(t,x)$ and population range $[g(t), h(t)]$, we demonstrate that the Allee effects can be eliminated if the species maintains its population density at a suitable level at the range boundary by advancing or retreating the fronts. It is proved that with such a strategy at the range edge the species can invade the environment successfully with all admissible initial populations, exhibiting the dynamics of super invaders. Numerical simulations are used to help understand what happens if the population density level at the range boundary is maintained at other levels. If the invading cane toads in Australia used this strategy at the range boundary to become a super invader, then our results may explain why toads near the invading front evolve to have longer legs and run faster.

q-bio.PE↗

arXiv2024

Long-time dynamics of a competition model with nonlocal diffusion and free boundaries: Vanishing and spreading of the invader

In this work, we investigate the long-time dynamics of a two species competition model of Lotka-Volterra type with nonlocal diffusions. One of the species, with density $v(t,x)$, is assumed to be a native in the environment (represented by the real line $\R$), while the other species, with density $u(t,x)$, is an invading species which invades the territory of $v$ with two fronts, $x=g(t)$ on the left and $x=h(t)$ on the right. So the population range of $u$ is the evolving interval $[g(t), h(t)]$ and the reaction-diffusion equation for $u$ has two free boundaries, with $g(t)$ decreasing in $t$ and $h(t)$ increasing in $t$, and the limits $h_\infty:=h(\infty)\leq \infty$ and $g_\infty:=g(\infty)\geq -\infty$ thus always exist. We obtain detailed descriptions of the long-time dynamics of the model according to whether $h_\infty-g_\infty$ is $\infty$ or finite. In the latter case, we reveal in what sense the invader $u$ vanishes in the long run and $v$ survives the invasion, while in the former case, we obtain a rather satisfactory description of the long-time asymptotic limit for both $u(t,x)$ and $v(t,x)$ when a certain parameter $k$ in the model is less than 1. This research is continued in a separate work, where sharp criteria are obtained to distinguish the case $h_\infty-g_\infty=\infty$ from the case $h_\infty-g_\infty$ is finite, and new phenomena are revealed for the case $k\geq 1$. The techniques developed in this paper should have applications to other models with nonlocal diffusion and free boundaries.

math.AP↗

arXiv2024

Long-time dynamics of a competition model with nonlocal diffusion and free boundaries: Chances of successful invasion

This is a continuation of our work \cite{dns-part1} to investigate the long-time dynamics of a two species competition model of Lotka-Volterra type with nonlocal diffusions, where the territory (represented by the real line $\R$) of a native species with density $v(t,x)$, is invaded by a competitor with density $u(t,x)$, via two fronts, $x=g(t)$ on the left and $x=h(t)$ on the right. So the population range of $u$ is the evolving interval $[g(t), h(t)]$ and the reaction-diffusion equation for $u$ has two free boundaries, with $g(t)$ decreasing in $t$ and $h(t)$ increasing in $t$. Let $h_\infty:=h(\infty)\leq \infty$ and $g_\infty:=g(\infty)\geq -\infty$. In \cite{dns-part1}, we obtained detailed descriptions of the long-time dynamics of the model according to whether $h_\infty-g_\infty$ is $\infty$ or finite. In the latter case, we demonstrated in what sense the invader $u$ vanishes in the long run and $v$ survives the invasion, while in the former case, we obtained a rather satisfactory description of the long-time asymptotic limits of $u(t,x)$ and $v(t,x)$ when the parameter $k$ in the model is less than 1. In the current paper, we obtain sharp criteria to distinguish the case $h_\infty-g_\infty=\infty$ from the case $h_\infty-g_\infty$ is finite. Moreover, for the case $k\geq 1$ and $u$ is a weak competitor, we obtain biologically meaningful conditions that guarantee the vanishing of the invader $u$, and reveal chances for $u$ to invade successfully. In particular, we demonstrate that both $h_\infty=\infty=-g_\infty$ and $h_\infty=\infty$ but $g_\infty$ is finite are possible; the latter seems to be the first example for this kind of population models, with either local or nonlocal diffusion.

math.AP↗

arXiv2023

Rate of accelerated expansion of the epidemic region in a nonlocal epidemic model with free boundaries

This paper is concerned with the long-time dynamics of an epidemic model whose diffusion and reaction terms involve nonlocal effects described by suitable convolution operators, and the epidemic region is represented by an evolving interval enclosed by the free boundaries in the model. In Wang and Du \cite{WangDu-JDE}, it was shown that the model is well-posed, and its long-time dynamical behaviour is governed by a spreading-vanishing dichotomy. The spreading speed was investigated in a subsequent work of Wang and Du \cite{WangDu-DCDS-A}, where a threshold condition for the diffusion kernels $J_1$ and $J_2$ was obtained, such that the asymptotic spreading speed is finite precisely when this condition is satisfied. In this paper, we examine the case that this threshold condition is not satisfied, which leads to accelerated spreading; for some typical classes of kernel functions, we determine the precise rate of accelerated expansion of the epidemic region by constructing delicate upper and lower solutions.

math.AP↗

arXiv2021

The high dimensional Fisher-KPP nonlocal diffusion equation with free boundary and radial symmetry

We study the radially symmetric high dimensional Fisher-KPP nonlocal diffusion equation with free boundary, and reveal some fundamental differences from its one dimensional version considered in \cite{cdjfa} recently. Technically, this high dimensional problem is much more difficult to treat since it involves two kernel functions which arise from the original kernel function $J(|x|)$ in rather implicit ways. By introducing new techniques, we are able to determine the long-time dynamics of the model, including firstly finding the threshold condition on the kernel function that governs the onset of accelerated spreading, and the determination of the spreading speed when it is finite. Moreover, for two important classes of kernel functions, sharp estimates of the spreading profile are obtained. More precisely, for kernel functions with compact support, we show that logarithmic shifting occurs from the finite wave speed propagation, which is strikingly different from the one dimension case; for kernel functions $J(|x|)$ behaving like $|x|^{-β}$ for $x\in\R^N$ near infinity, we obtain the rate of accelerated spreading when $β\in (N, N+1]$, which is the exact range of $β$ where accelerated spreading is possible. These sharp estimates are obtained by constructing subtle upper and lower solutions, based on careful analysis of the involved kernel functions.

math.AP↗

arXiv2020

Semi-wave, traveling wave and spreading speed for monostable cooperative systems with nonlocal diffusion and free boundaries

We consider a class of cooperative reaction-diffusion systems with free boundaries in one space dimension, where the diffusion terms are nonlocal, given by integral operators involving suitable kernel functions, and they are allowed not to appear in some of the equations in the system. The problem is monostable in nature, resembling the well known Fisher-KPP equation. Such a system covers various models arising from mathematical biology, with the Fisher-KPP equation as the simplest special case, where a "spreading-vanishing" dichotomy is known to govern the long time dynamical behaviour. The question of spreading speed is widely open for such systems except for the scalar case. In this paper, we develop a systematic approach to determine the spreading profile of the system, and obtain threshold conditions on the kernel functions which decide exactly when the spreading has finite speed, or infinite speed (accelerated spreading). This relies on a rather complete understanding of both the associated semi-waves and traveling waves. When the spreading speed is finite, we show that the speed is determined by a particular semi-wave, and obtain sharp estimates of the semi-wave profile and the spreading speed. For kernel functions that behave like $|x|^{-γ}$ near infinity, we are able to obtain better estimates of the spreading speed for both the finite speed case, and the infinite speed case, which appear to be the first for this kind of free boundary problems, even for the special Fisher-KPP equation.

math.AP↗

arXiv2020

Global stability of spatially nonhomogeneous steady state solution in a diffusive Holling-Tanner predator-prey model

The global stability of the nonhomogeneous positive steady state solution to a diffusive Holling-Tanner predator-prey model in a heterogeneous environment is proved by using a newly constructed Lyapunov function and estimates of nonconstant steady state solutions. The techniques developed here can be adapted for other spatially heterogeneous consumer-resource models.

math.AP↗

arXiv2020

Approximation of random diffusion equation by nonlocal diffusion equation in free boundary problems of one space dimension

We show how the Stefan type free boundary problem with random diffusion in one space dimension can be approximated by the corresponding free boundary problem with nonlocal diffusion. The approximation problem is a slightly modified version of the nonlocal diffusion problem with free boundaries considered in [4,8]. The proof relies on the introduction of several auxiliary free boundary problems and constructions of delicate upper and lower solutions for these problems. As usual, the approximation is achieved by choosing the kernel function in the nonlocal diffusion term of the form $J_ε(x)=\frac 1εJ(\frac xε)$ for small $ε>0$, where $J(x)$ has compact support. We also give an estimate of the error term of the approximation by some positive power of $ε$.

math.AP↗

arXiv2019

Global stability of nonhomogeneous equilibrium solution for the diffusive Lotka-Volterra competition model

A diffusive Lotka-Volterra competition model is considered for the combined effect of spatial dispersal and spatial variations of resource on the population persistence and exclusion. First it is shown that in a two-species system in which the diffusion coefficients, resource functions and competition rates are all spatially heterogeneous, the positive equilibrium solution is globally asymptotically stable when it exists. Secondly the existence and global asymptotic stability of the positive and semi-trivial equilibrium solutions are obtained for the model with arbitrary number of species under the assumption of spatially heterogeneous resource distribution. A new Lyapunov functional method is developed to prove the global stability of a non-constant equilibrium solution in heterogeneous environment.

math.AP↗