Searcharxiv⌕ Search

arXiv subjects

Mario Fuest

Publications and source records attributed to Mario Fuest.

At least 19 recordsLinked to original sources
arXiv2026

Absence of critical mass phenomena in one-dimensional critical quasilinear Keller-Segel systems

We consider the Neumann initial boundary value problem associated to the chemotaxis system \begin{align}\label{prob:abstract}\tag{$\star$} \begin{cases} u_t = \big((u+1)^{m-1} u_x - u(u+1)^m v_x\big)_x & \text{in $(0, 1) \times (0, \infty)$}, \\ v_t = v_{xx} - v + u, &\text{in $(0, 1) \times (0, \infty)$}, \end{cases} \end{align} where $m \in \mathbb R$ is a given parameter. The relation between diffusion and taxis sensitivity is critical since the ratio $u(u+1)^m/(u+1)^{m-1}$ grows like $u^{2/n}$ for large $u$ with $n = \dim((0, 1)) = 1$. Nonetheless, we show that there is no critical mass phenomenon if $m \le -1$; that is, in that case all solutions emanating from suitably regular initial data are globally bounded. For certain parabolic-elliptic simplifications of \eqref{prob:abstract}, we obtain the same conclusion for all $m \in (-\infty, -1] \cup (0, \infty)$ and even for all $m \in \mathbb R$ if the initial datum is additionally assumed to be monotone. This stands in contrast to critical mass phenomena known to occur for critical quasilinear Keller-Segel systems considered in higher-dimensional domains. Accordingly, we make use of several special features of the one-dimensional setting such as the boundedness of the energy functional from below, the embedding $W^{1, n} \hookrightarrow L^\infty$, and the fact that the mass accumulation function solves a spatially non-degenerate parabolic equation.

math.AP↗
arXiv2025

Existence, uniqueness, and long-time asymptotic behavior of regular solutions in multidimensional thermoelasticity

We study a simplified nonlinear thermoelasticity model on two- and three-dimensional tori. A novel functional involving the Fisher information associated with temperature is introduced, extending the previous one-dimensional approach from the first two authors (SIAM J.\ Math.\ Anal.\ \textbf{55} (2023), 7024--7038)) to higher dimensions. Using this functional, we prove global/local existence of unique regular solutions for small/large initial data. Furthermore, we analyze the asymptotic behavior as time approaches infinity and show that the temperature stabilizes to a constant state, while the displacement naturally decomposes into two distinct components: a divergence-free part oscillating indefinitely according to a homogeneous wave equation and a curl-free part converging to zero. Analogous results for the Lam\'e operator are also stated.

math.AP↗
arXiv2025

Shrinking vs. expanding: the evolution of spatial support in degenerate Keller-Segel systems

We consider radially symmetric solutions of the degenerate Keller-Segel system \begin{align*} \begin{cases} \partial_t u=\nabla\cdot (u^{m-1}\nabla u - u\nabla v),\\ 0=\Delta v -\mu +u,\quad\mu =\frac{1}{|\Omega|}\int_\Omega u, \end{cases} \end{align*} in balls $\Omega\subset\mathbb R^n$, $n\ge 1$, where $m>1$ is arbitrary. Our main result states that the initial evolution of the positivity set of $u$ is essentially determined by the shape of the (nonnegative, radially symmetric, H\"older continuous) initial data $u_0$ near the boundary of its support $\overline{B_{r_1}(0)}\subsetneq\Omega$: It shrinks for sufficiently flat and expands for sufficiently steep $u_0$. More precisely, there exists an explicit constant $A_{\mathrm{crit}} \in (0, \infty)$ (depending only on $m, n, R, r_1$ and $\int_\Omega u_0$) such that if \begin{align*} u_0(x)\le A(r_1-|x|)^\frac{1}{m-1} \qquad \text{for all $|x|\in(r_0, r_1)$ and some $r_0\in(0,r_1)$ and $A 0$ and $\zeta>0$ such that $\sup\{\, |x| \mid x \in \operatorname{supp} u(\cdot, t)\,\}\le r_1 -\zeta t$ for all $t\in(0, T)$, while if \begin{align*} u_0(x)\ge A(r_1-|x|)^\frac{1}{m-1} \qquad \text{for all $|x|\in(r_0, r_1)$ and some $r_0 \in (0, r_1)$ and $A>A_{\mathrm{crit}}$}, \end{align*} then we can find $T>0$ and $\zeta>0$ such that $\sup\{\, |x| \mid x \in \operatorname{supp} u(\cdot, t)\,\}\ge r_1 +\zeta t$ for all $t\in(0, T)$.

math.AP↗
arXiv2024

Global solvability of a model for tuberculosis granuloma formation

We discuss a nonlinear system of partial differential equations modelling the formation of granuloma during tuberculosis infections and prove the global solvability of the homogeneous Neumann problem for \begin{align*} \begin{cases} u_t = D_u \Delta u - \chi_u \nabla \cdot (u \nabla v) - \gamma_u uv - \delta_u u + \beta_u, \\ v_t = D_v \Delta v + \rho_v v - \gamma_v uv + \mu_v w,\\ w_t = D_w \Delta w + \gamma_w uv - \alpha_w wz - \mu_w w,\\ z_t = D_z \Delta z - \chi_z \nabla \cdot (z \nabla w) + \alpha_z f(w)z - \delta_z z \end{cases} \end{align*} in bounded domains in the classical and weak sense in the two- and three-dimensional setting, respectively. In order to derive suitable a~priori estimates, we study the evolution of the well-known energy functional for the chemotaxis-consumption system both for the $(u, v)$- and the $(z, w)$-subsystem. A key challenge compared to "pure" consumption systems consists of overcoming the difficulties raised by the additional, in part positive, terms in the second and third equations. This is inter alia achieved by utilising a dissipative term of the (quasi-)energy functional, which may just be discarded in simpler consumption systems.

math.AP↗
arXiv2024

Finite-time blow-up in fully parabolic quasilinear Keller-Segel systems with supercritical exponents

We examine the possibility of finite-time blow-up of solutions to the fully parabolic quasilinear Keller--Segel model \begin{align}\tag{$\star$}\label{prob:star} \begin{cases} u_t = \nabla \cdot ((u+1)^{m-1}\nabla u - u(u+1)^{q-1}\nabla v) & \text{in $\Omega \times (0, T)$}, \\ v_t = \Delta v - v + u & \text{in $\Omega \times (0, T)$} \end{cases} \end{align} in a ball $\Omega\subset \mathbb R^n$ with $n\geq 2$. Previous results show that unbounded solutions exist for all $m, q \in \mathbb R$ with $m-q<\frac{n-2}{n}$, which, however, are necessarily global in time if $q \leq 0$. It is expected that finite-time blow-up is possible whenever $q > 0$ but in the fully parabolic setting this has so far only been shown when $\max\{m, q\} \geq 1$. In the present paper, we substantially extend these findings. Our main results for the two- and three-dimensional settings state that \eqref{prob:star} admits solutions blowing up in finite time if \begin{align*} m-q<\frac{n-2}{n} \quad \text{and} \quad \begin{cases} q < 2m & \text{if } n = 2, \\ q < 2m - \frac23 \text{ or } m > \frac23 & \text{if } n = 3, \end{cases} \end{align*} that is, also for certain $m, q$ with $\max\{m, q\} < 1$. As a key new ingredient in our proof, we make use of (singular) pointwise upper estimates for $u$.

math.AP↗
arXiv2024

Upper estimates for the Hausdorff dimension of the temporal singular set in chemotaxis-fluid systems

The chemotaxis-fluid system \begin{align}\tag{$\star$}\label{prob:star} \begin{cases} n_t + u \cdot \nabla n = \Delta n - \nabla \cdot (n \nabla c), \\ c_t + u \cdot \nabla c = \Delta c - nc, \\ u_t + (u \cdot \nabla) u = \Delta u + \nabla P + n \nabla \Phi, \quad \nabla \cdot u = 0, \end{cases} \end{align} models aerobic bacteria interacting with a fluid via transportation and buoyancy. When posed on a three-dimensional, smoothly bounded, convex domain $\Omega$, \eqref{prob:star} complemented with suitable initial and boundary conditions is known to admit a global `weak energy solution', which recently has been shown to be smooth (after a redefinition on a set of measure $0$) in $\overline \Omega \times E$ for some countable union of open intervals $E$ with $|(0, \infty) \setminus E| = 0$. The present paper investigates further regularity properties of this solution and proves that ($E$ can be chosen such that) the $\frac12$-dimensional Hausdorff measure of $(0, \infty) \setminus E$ vanishes and thus that in particular its Hausdorff dimension is at most $\frac12$. As $\frac12$ has been the best known upper estimate for the Hausdorff dimension of the temporal singular set for the unperturbed Navier--Stokes equations for quite some time, this result is the best one can hope for \eqref{prob:star} without significant progress in the regularity theory of (homogeneous) Navier--Stokes equations.

math.AP↗
arXiv2024

Classical and generalized solutions of an alarm-taxis model

In bounded, spatially two-dimensional domains, the system \begin{equation*} \left\lbrace\begin{alignedat}{3} u_t &= d_1 \Delta u && &&+ u(\lambda_1 - \mu_1 u - a_1 v - a_2 w), \\ v_t &= d_2 \Delta v &&- \xi \nabla \cdot (v \nabla u) &&+ v(\lambda_2 - \mu_2 v + b_1 u - a_3 w),\\ w_t &= d_3 \Delta w &&- \chi \nabla \cdot (w \nabla (uv)) &&+ w(\lambda_3 - \mu_3 w + b_2 u + b_3 v), \end{alignedat}\right. \end{equation*} complemented with initial and homogeneous Neumann boundary conditions, models the interaction between prey (with density $u$), predator (with density $v$) and superpredator (with density $w$), which preys on both other populations. Apart from random motion and prey-tactical behavior of the primary predator, the key aspect of this system is that the secondary predator reacts to alarm calls of the prey, issued by the latter whenever attacked by the primary predator. We first show in the pure alarm-taxis model, i.e. if $\xi = 0$, that global classical solutions exist. For the full model (with $\xi > 0$), the taxis terms and the presence of the term $-a_2 uw$ in the first equation apparently hinder certain bootstrap procedures, meaning that the available regularity information is rather limited. Nonetheless, we are able to obtain global generalized solutions. An important technical challenge is to guarantee strong convergence of (weighted) gradients of the first two solution components in order to conclude that approximate solutions converge to a generalized solution of the limit problem.

math.AP↗
arXiv2024

Uniform $L^p$ estimates for solutions to the inhomogeneous 2D Navier-Stokes equations and application to a chemotaxis-fluid system with local sensing

The chemotaxis-Navier-Stokes system \begin{equation*}\label{1} \left\{ \begin{array}{rcl} n_t+u\cdot\nabla n &=& \Delta \big(n c^{-\alpha} \big), \\[1mm] c_t+ u\cdot\nabla c &=& \Delta c -nc,\\[1mm] u_t + (u\cdot\nabla) u &=&\Delta u+\nabla P + n\nabla\Phi, \qquad \nabla\cdot u=0, \end{array} \right. \end{equation*} modelling the behavior of aerobic bacteria in a fluid drop, is considered in a smoothly bounded domain $\Omega \subset \mathbb R^2$. For all $\alpha > 0$ and all sufficiently regular $\Phi$, we construct global classical solutions and thereby extend recent results for the fluid-free analogue to the system coupled to a Navier-Stokes system. As a crucial new challenge, our analysis requires a priori estimates for $u$ at a point in the proof when knowledge about $n$ is essentially limited to the observation that the mass is conserved. To overcome this problem, we also prove new uniform-in-time $L^p$ estimates for solutions to the inhomogeneous Navier-Stokes equations merely depending on the space-time $L^2$ norm of the force term raised to an arbitrary small power.

math.AP↗
arXiv2023

A cross-diffusion system modelling rivaling gangs: global existence of bounded solutions and FCT stabilization for numerical simulation

For the gang territoriality model \begin{align*} \begin{cases} u_t = D_u \Delta u + \chi_u \nabla \cdot (u \nabla w), \\ v_t = D_v \Delta v + \chi_v \nabla \cdot (v \nabla z), \\ w_t = -w + \frac{v}{1+v}, \\ z_t = -z + \frac{u}{1+u}, \end{cases} \end{align*} where $u$ and $v$ denote the densities of two rivaling gangs which spray graffiti (with densities $z$ and $w$, respectively) and partially move away from the other gang's graffiti, we construct global, bounded classical solutions. By making use of quantitative global estimates, we prove that these solutions converge to homogeneous steady states if $\|u_0\|_{L^\infty(\Omega)}$ and $\|v_0\|_{L^\infty(\Omega)}$ are sufficiently small. Moreover, we perform numerical experiments which show that for different choices of parameters, the system may become diffusion- or convection-dominated, where in the former case the solutions converge toward constant steady states while in the later case nontrivial asymptotic behavior such as segregation is observed. In order to perform these experiments, we apply a nonlinear finite element flux-corrected transport method (FEM-FCT) which is positivity-preserving. Then, we treat the nonlinearities in both the system and the proposed nonlinear scheme simultaneously using fixed-point iteration.

math.AP↗
arXiv2023

Corners and collapse: Some simple observations concerning critical masses and boundary blow-up in the fully parabolic Keller-Segel system

Our main result shows that the mass $2\pi$ is critical for the minimal Keller-Segel system \begin{align}\label{prob:abstract}\tag{$\star$} \begin{cases} u_t = \Delta u - \nabla \cdot (u \nabla v), \\ v_t = \Delta v - v + u, \end{cases} \end{align} considered in a quarter disc $\Omega = \{\,(x_1, x_2) \in \mathbb R : x_1 > 0, x_2 > 0, x_1^2 + x_2^2 < R^2\,\}$, $R > 0$, in the following sense: For all reasonably smooth nonnegative initial data $u_0, v_0$ with $\int_\Omega u_0 < 2\pi$, there exists a global classical solution to the Neumann initial boundary value problem associated to \eqref{prob:abstract}, while for all $m > 2 \pi$ there exist nonnegative initial data $u_0, v_0$ with $\int_\Omega u_0 = m$ so that the corresponding classical solution of this problem blows up in finite time. At the same time, this gives an example of boundary blow-up in \eqref{prob:abstract}. Up to now, precise values of critical masses had been observed in spaces of radially symmetric functions or for parabolic-elliptic simplifications of \eqref{prob:abstract} only.

math.AP↗
arXiv2023

On the existence of global solutions for the 3D chemorepulsion system

In this paper, we give sufficient conditions for global-in-time existence of classical solutions for the fully parabolic chemorepulsion system posed on a convex, bounded three-dimensional domain. Our main result establishes global-in-time existence of regular nonnegative solutions provided that $\nabla\sqrt{u} \in L^4(0, T; L^2(\Omega))$. Our method is related to the Bakry--\'Emery calculation and appears to be new in this context.

math.AP↗
arXiv2023

Critical mass phenomena in higher dimensional quasilinear Keller-Segel systems with indirect signal production

In this paper, we deal with quasilinear Keller--Segel systems with indirect signal production, $$\begin{cases} u_t = \nabla \cdot ((u+1)^{m-1}\nabla u) - \nabla \cdot (u \nabla v), &x \in \Omega,\ t> 0,\\ 0 = \Delta v - \mu(t) + w, &x \in \Omega,\ t> 0,\\ w_t + w = u, &x \in \Omega,\ t> 0, \end{cases}$$ complemented with homogeneous Neumann boundary conditions and suitable initial conditions, where $\Omega\subset\mathbb R^n$ $(n\ge3)$ is a bounded smooth domain, $m\ge1$ and $$\mu(t) := \frac{1}{|\Omega|} w(\cdot, t) \qquad\mbox{for}\ t>0.$$ We show that in the case $m\ge2-\frac{2}{n}$, there exists $M_c>0$ such that if either $m>2-\frac{2}{n}$ or $\int_\Omega u_0 2^\frac{n}{2}n^{n-1}\omega_n$, then there exist radially symmetric initial data such that $\int_\Omega u_0 = M$ and the solution blows up in finite or infinite time, where the blow-up time is infinite if $m=2-\frac2n$. In particular, if $m=2-\frac{2}{n}$ there is a critical mass phenomenon in the sense that $\inf\left\{M > 0 : \exists u_0 \text{ with } \int_\Omega u_0 = M \text{ such that the corresponding solution blows up in infinite time}\right\}$ is a finite positive number.

math.AP↗
arXiv2022

Possible points of blow-up in chemotaxis systems with spatially heterogeneous logistic source

We discuss the influence of possible spatial inhomogeneities in the coefficients of logistic source terms in parabolic-elliptic chemotaxis-growth systems of the form \begin{align*} u_t &= \Delta u - \nabla\cdot(u\nabla v) + \kappa(x)u-\mu(x)u^2, 0 &= \Delta v - v + u \end{align*} in smoothly bounded domains $\Omega\subset\mathbb{R}^2$. Assuming that the coefficient functions satisfy $\kappa,\mu\in C^0(\overline{\Omega})$ with $\mu\geq0$ we prove that finite-time blow-up of the classical solution can only occur in points where $\mu$ is zero, i.e.\ that the blow-up set $\mathcal{B}$ is contained in \begin{align*} \big\{x\in\overline{\Omega}\mid\mu(x)=0\big\}. \end{align*} Moreover, we show that whenever $\mu(x_0)>0$ for some $x_0\in\overline{\Omega}$, then one can find an open neighbourhood $U$ of $x_0$ in $\overline{\Omega}$ such that $u$ remains bounded in $U$ throughout evolution.

math.AP↗
arXiv2022

Global existence of classical solutions and numerical simulations of a cancer invasion model

In this paper, we study a cancer invasion model both theoretically and numerically. The model is a nonstationary, nonlinear system of three coupled partial differential equations modeling the motion of cancer cells, degradation of the extracellular matrix, and certain enzymes. We first establish existence of global classical solutions in both two- and three-dimensional bounded domains, despite the lack of diffusion of the matrix-degrading enzymes and corresponding regularizing effects in the analytical treatment. Next, we give a weak formulation and apply finite differences in time and a Galerkin finite element scheme for spatial discretization. The overall algorithm is based on a fixed-point iteration scheme. In order to substantiate our theory and numerical framework, several numerical simulations are carried out in two and three spatial dimensions.

math.NA↗
arXiv2022

Strong convergence of weighted gradients in parabolic equations and applications to global generalized solvability of cross-diffusive systems

In the first part of the present paper, we show that strong convergence of $(v_{0 \varepsilon})_{\varepsilon \in (0, 1)}$ in $L^1(\Omega)$ and weak convergence of $(f_{\varepsilon})_{\varepsilon \in (0, 1)}$ in $L_{\textrm{loc}}^1(\overline \Omega \times [0, \infty))$ not only suffice to conclude that solutions to the initial boundary value problem \begin{align*} \begin{cases} v_{\varepsilon t} = \Delta v_\varepsilon + f_\varepsilon(x, t) & \text{in $\Omega \times (0, \infty)$}, \\ \partial_\nu v_\varepsilon = 0 & \text{on $\partial \Omega \times (0, \infty)$}, \\ v_\varepsilon(\cdot, 0) = v_{0 \varepsilon} & \text{in $\Omega$}, \end{cases} \end{align*} which we consider in smooth, bounded domains $\Omega$, converge to the unique weak solution of the limit problem, but that also certain weighted gradients of $v_\varepsilon$ converge strongly in $L_{\textrm{loc}}^2(\overline \Omega \times [0, \infty))$ along a subsequence. We then make use of these findings to obtain global generalized solutions to various cross-diffusive systems. Inter alia, we establish global generalized solvability of the system \begin{align*} \begin{cases} u_t = \Delta u - \chi \nabla \cdot (\frac{u}{v} \nabla v) + g(u), \\ v_t = \Delta v - uv, \end{cases} \end{align*} where $\chi > 0$ and $g \in C^1([0, \infty))$ are given, merely provided that ($g(0) \geq 0$ and) $-g$ grows superlinearily. This result holds in all space dimensions and does neither require any symmetry assumptions nor the smallness of certain parameters. Thereby, we expand on a corresponding result for quadratically growing $-g$ proved by Lankeit and Lankeit (Nonlinearity, 32(5):1569--1596, 2019).

math.AP↗
arXiv2021

Unboundedness phenomenon in a model of urban crime

We show that spatial patterns ("hotspots") may form in the crime model \begin{equation} \left\{\; \begin{aligned} u_{t} &= \tfrac{1}{\varepsilon}\Delta u - \tfrac{\chi}{\varepsilon} \nabla \cdot \left(\tfrac{u}{v} \nabla v \right) - \varepsilon uv, \\ v_{t} &= \Delta v - v + u v, \end{aligned} \right. \end{equation} which we consider in $\Omega = B_R(0) \subset \mathbb R^n$, $R > 0$, $n \geq 3$ with $\varepsilon > 0$, $\chi > 0$ and initial data $u_0$, $v_0$ with sufficiently large initial mass $m := \int_\Omega u_0$. More precisely, for each $T > 0$ and fixed $\Omega$, $\chi$ and (large) $m$, we construct initial data $v_0$ exhibiting the following unboundedness phenomenon: Given any $M>0$, we can find $\varepsilon > 0$ such that the first component of the associated maximal solution becomes larger than $M$ at some point in $\Omega$ before the time $T$. Since the $L^1$ norm of $u$ is decreasing, this implies that some heterogeneous structure must form. We do this by first constructing classical solutions to the nonlocal scalar problem \[ w_t = \Delta w + m \frac{w^{\chi+1}}{\int_\Omega w^\chi} \] from the solutions to the crime model by taking the limit $\varepsilon \searrow 0$ under the assumption that the unboundedness phenomenon explicitly does not occur on some interval $(0,T)$. We then construct initial data for this scalar problem leading to blow-up before time $T$. As solutions to the scalar problem are unique, this proves our central result by contradiction.

math.AP↗