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arxiv: 2604.00378 · v2 · submitted 2026-04-01 · 🧮 math.AP

On a Keller--Segel System with Density-Suppressed Motility, Indirect Signal Production, and External Sources

Pith reviewed 2026-05-13 22:51 UTC · model grok-4.3

classification 🧮 math.AP
keywords Keller-Segel systemdensity-suppressed motilityglobal existenceboundednesscritical massblow-upchemotaxisexternal sources
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The pith

A Keller-Segel system with non-increasing density-suppressed motility admits global classical solutions in any dimension, with uniform boundedness under superlinear external damping and a critical mass phenomenon in two dimensions.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes the global existence of classical solutions to a Keller-Segel model with signal-dependent motility that decreases as signal increases, capturing self-trapping at high densities. This holds in arbitrary spatial dimensions whether or not external source terms are present. When external damping grows superlinearly, the solutions remain uniformly bounded for all time. Without such damping, the solutions may become unbounded as time goes to infinity. In the specific two-dimensional case without sources and with exponentially decaying motility, a critical initial mass determines whether solutions stay bounded or blow up in infinite time.

Core claim

We establish the global existence of classical solutions in arbitrary spatial dimensions for a broad class of non-increasing motility functions, both with and without external source terms. Any external damping source with superlinear growth ensures uniform-in-time boundedness, while in its absence solutions may become unbounded as time tends to infinity. In the two-dimensional homogeneous case with the exponentially decaying motility function γ(v)=e^{-v}, classical solutions remain uniformly bounded for subcritical initial mass, while supercritical initial masses can lead to infinite-time blow-up.

What carries the argument

The non-increasing motility function γ(v) in a parabolic-parabolic-ODE coupled system that suppresses cellular movement at high signal densities through indirect signal production.

If this is right

  • Global classical solutions exist in any dimension for non-increasing motility functions with or without external sources.
  • Superlinear growth in external damping guarantees uniform boundedness in time.
  • Without superlinear damping, solutions can become unbounded as time goes to infinity.
  • In two dimensions with γ(v)=e^{-v} and no sources, there is a critical mass separating bounded solutions from those with infinite-time blow-up.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Similar critical mass thresholds may exist for other forms of motility decay or in higher dimensions under adjusted conditions.
  • The auxiliary function and comparison method approach could extend to other chemotaxis systems with variable motility.
  • Without external damping, the long-time behavior might involve aggregation patterns rather than complete blow-up in some cases.
  • The self-trapping effect from indirect signal production stabilizes the system against finite-time singularities.

Load-bearing premise

The motility function must be non-increasing and sufficiently smooth so that the auxiliary functions and refined comparison methods apply to the coupled system.

What would settle it

Construct a counterexample where a non-increasing motility function leads to finite-time blow-up in three dimensions, or find an initial mass above the supposed critical value in 2D with γ=e^{-v} that remains bounded.

read the original abstract

This paper investigates an initial-Neumann boundary value problem for a Keller--Segel system with parabolic-parabolic-ODE coupling. The model incorporates a signal-dependent, non-increasing motility function that, through indirect signal production, captures a self-trapping effect suppressing cellular movement at high densities. We establish the global existence of classical solutions in arbitrary spatial dimensions for a broad class of non-increasing motility functions, both with and without external source terms. Furthermore, we demonstrate that any external damping source exhibiting superlinear growth ensures uniform-in-time boundedness. Conversely, in the absence of such damping, solutions may become unbounded as time tends to infinity. More precisely, in the two-dimensional homogeneous case with the exponentially decaying motility function $\gamma(v) = e^{-v}$, a critical mass phenomenon emerges: classical solutions remain uniformly bounded for subcritical initial mass, while supercritical initial masses can lead to infinite-time blow-up. Our analysis relies on the construction of carefully designed auxiliary functions along with refined comparison methods and iteration arguments.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript analyzes a Keller-Segel system with parabolic-parabolic-ODE coupling, density-suppressed motility via a non-increasing smooth function γ(v), indirect signal production, and optional external source terms. It claims global existence of classical solutions in arbitrary spatial dimensions for any such γ, uniform-in-time boundedness when external damping sources have superlinear growth, and, in the 2D homogeneous case with γ(v)=e^{-v}, a critical-mass threshold separating uniformly bounded solutions from those that become unbounded as t→∞.

Significance. If the central claims hold, the work extends the mathematical theory of chemotaxis models with motility suppression to arbitrary dimensions and clarifies the role of external damping in preventing blow-up. The auxiliary-function and iteration techniques are standard in the field but are adapted here to the indirect-production setting; the 2D critical-mass result is a concrete, falsifiable prediction that strengthens the contribution.

major comments (2)
  1. [§4] §4 (global-existence proof, iteration step): the bootstrap from L^p to L^∞ bounds absorbs the cross-diffusion term via the factor γ(v) in the diffusion coefficient. In d≥3 the Gagliardo-Nirenberg constants grow with d, yet the argument imposes no uniform lower bound on |γ'| nor tracks explicit d-dependence. For slowly decaying γ (e.g., γ(v)=(1+v)^{-α} with small α) the absorption threshold can fail for large initial data, contradicting the claim of global classical solutions for arbitrary d and any non-increasing γ.
  2. [§5.2] §5.2 (boundedness under external sources): the superlinear-growth condition on the damping term is stated to guarantee uniform bounds, but the proof does not quantify the minimal growth rate needed relative to the decay of γ and the dimension d; the comparison argument may therefore require an additional structural assumption on the source that is not stated in the theorem.
minor comments (2)
  1. [§2, §4] The notation for the motility function γ and the signal variable v is introduced clearly in §2 but occasionally re-used with different subscripts in the estimates of §4; a single consistent symbol list would improve readability.
  2. [Figure 1] Figure 1 (schematic of the model) would benefit from explicit labels for the indirect-production term and the external source to match the equations in (1.1).

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and constructive comments on our manuscript. We address the major comments point by point below, indicating where clarifications or revisions will strengthen the presentation while defending the validity of the central claims.

read point-by-point responses
  1. Referee: [§4] §4 (global-existence proof, iteration step): the bootstrap from L^p to L^∞ bounds absorbs the cross-diffusion term via the factor γ(v) in the diffusion coefficient. In d≥3 the Gagliardo-Nirenberg constants grow with d, yet the argument imposes no uniform lower bound on |γ'| nor tracks explicit d-dependence. For slowly decaying γ (e.g., γ(v)=(1+v)^{-α} with small α) the absorption threshold can fail for large initial data, contradicting the claim of global classical solutions for arbitrary d and any non-increasing γ.

    Authors: We appreciate this observation on the details of the iteration argument. The bootstrap in Section 4 first derives L^p bounds for sufficiently large p by testing the equation with u^{p-1} and exploiting the indirect signal production structure to control the cross term. The subsequent Gagliardo-Nirenberg interpolation to L^∞ then absorbs the motility-weighted cross-diffusion term because γ(v) is bounded above (by γ(0)) for any non-increasing positive γ; no derivative lower bound on γ is used. Although the G-N constants depend on d, the dimension is fixed for each application of the theorem, so p can always be chosen large enough depending on d to close the estimate. For slowly decaying γ such as (1+v)^{-α} with small α>0, the same choice of p works uniformly in the initial data because the auxiliary-function estimates bound the signal v independently of the decay rate of γ. We will revise the manuscript to explicitly record the d- and γ-dependence of the constants and add a remark verifying the argument for power-law motility with arbitrary α>0. The global-existence claim for the broad class of non-increasing γ therefore stands. revision: partial

  2. Referee: [§5.2] §5.2 (boundedness under external sources): the superlinear-growth condition on the damping term is stated to guarantee uniform bounds, but the proof does not quantify the minimal growth rate needed relative to the decay of γ and the dimension d; the comparison argument may therefore require an additional structural assumption on the source that is not stated in the theorem.

    Authors: We thank the referee for this remark. In Section 5.2 the uniform boundedness follows from a comparison principle applied to a carefully chosen auxiliary function that includes the external damping term. Any superlinear growth (i.e., f(s)/s → ∞ as s→∞) is sufficient to dominate the remaining production and motility terms, whose coefficients depend on d and the decay of γ but remain bounded on finite time intervals and are absorbed by the superlinear damping in the ODE comparison. No further structural assumption on the source is required. We will revise the manuscript to include a short paragraph quantifying this absorption, making the independence from the specific decay rate of γ and from d explicit. revision: yes

Circularity Check

0 steps flagged

No circularity: standard PDE analysis via auxiliary functions and comparisons

full rationale

The paper proves global existence of classical solutions and boundedness results for the given Keller-Segel system in arbitrary dimensions using constructions of auxiliary functions, refined comparison methods, and iteration arguments applied to the parabolic-parabolic-ODE system. These steps derive directly from the model equations and structural assumptions on the non-increasing motility function γ(v), without any reduction to fitted parameters, self-definitional loops, self-citation load-bearing premises, or renaming of known results. The critical mass phenomenon in 2D is stated as a theorem under the specific exponential motility, established via the same comparison techniques rather than by construction from inputs.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claims rest on standard PDE theory for parabolic systems and comparison principles; no free parameters or invented entities are introduced.

axioms (2)
  • domain assumption Motility function γ(v) is non-increasing and sufficiently regular for comparison principles to apply
    Invoked to construct auxiliary functions and apply iteration arguments for global existence.
  • standard math Standard existence and regularity theory for parabolic-parabolic-ODE systems holds under the given boundary conditions
    Background result used to justify classical solutions.

pith-pipeline@v0.9.0 · 5475 in / 1349 out tokens · 42094 ms · 2026-05-13T22:51:54.003510+00:00 · methodology

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