Stability vs. instability of singular steady states in the parabolic-elliptic Keller-Segel system on $\mathbb R^n$

TL;DR

This paper investigates the stability and instability of a singular steady state in the Keller-Segel system across various dimensions, revealing dimension-dependent behaviors and conditions for blow-up or stability.

Contribution

It extends the understanding of stability properties of the singular steady state to all dimensions n ≥ 3, including new results for 3 ≤ n ≤ 9.

Findings

Infinite-time blow-up for initial data less concentrated than u_* in high dimensions.

Instability of u_* for dimensions 3 to 9.

Existence of a bounded absorbing set for radial trajectories less concentrated than u_*.

Abstract

The Cauchy problem in $\mathbb R^n$ is considered for \begin{eqnarray*} \left\{ \begin{array}{l} u_t = \Delta u - \nabla \cdot (u\nabla v),\\ 0 = \Delta v + u. \end{array} \right. \end{eqnarray*} For each $n\ge 10$, a statement on stability and attractiveness of the singular steady state given by \[ u_\star(x):=\frac{2(n-2)}{|x|^2},\qquad x\in\mathbb R^n\setminus\{0\}, \] is derived within classes of nonnegative radial solutions emanating from initial data less concentrated than $u_\star$. In particular, for any such $n$ it is shown that infinite-time blow-up occurs for all radial initial data which are less concentrated than $u_\star$ and satisfy \[ u_0(x) \ge \frac{2(n-2)}{|x|^2} - \frac{C}{|x|^{2+\theta}}\qquad \mbox{for all } x\in \mathbb R^n\setminus B_1(0) \] with some $C>0$ and some $\theta>\frac{n-2+\sqrt{(n-2)(n-10)}}{2}$. This is complemented by a result which, in the case when $3\le n \le 9$, asserts instability of $u_\star$ as well as the existence of a bounded absorbing set for all radial trajectories initially less concentrated than $u_\star$. In particular, previous knowledge on stability properties of $u_\star$, as having been gained for $n\ge 11$ in [24], is thereby extended to any dimension $n\ge 3$.