A degenerate chemotaxis system with flux limitation: Finite-time blow-up
This paper is concerned with radially symmetric solutions of the parabolic-elliptic version of the Keller-Segel system with flux limitation, as given by \begin{equation} \left\{ \begin{array}{l} \displaystyle u_t=\nabla \cdot \Big(\frac{u\nabla u}{\sqrt{u^2+|\nabla u|^2}}\Big) - χ\, \nabla \cdot \Big(\frac{u\nabla v}{\sqrt{1+|\nabla v|^2}}\Big), \\[1mm] 0=Δv - μ+ u, \end{array} \right. \qquad \qquad (\star) \end{equation} under the initial condition $u|_{t=0}=u_0>0$ and no-flux boundary conditions in a ball $Ω\subset R^n$, where $χ>0$ and $μ:=\frac{1}{|Ω|} \int_Ωu_0$. A previous result [3] has asserted global existence of bounded classical solutions for arbitrary positive radial initial data $u_0\in C^3(\barΩ)$ when either $n\ge 2$ and $χ<1$, or $n=1$ and $ \int_Ωu_0<\frac{1}{\sqrt{(χ^2-1)_+}}$. This present paper shows that these conditions are essentially optimal: Indeed, it is shown that if the taxis coefficient is large enough in the sense that $χ>1$, then for any choice of \begin{equation} \left\{ \begin{array}{ll} m>\frac{1}{\sqrt{χ^2-1}} \quad & \mbox{if } n=1, \\[2mm] m>0 \mbox{ is arbitrary } \quad & \mbox{if } n\ge 2, \end{array} \right. \end{equation} there exist positive initial data $u_0\in C^3(\barΩ)$ satisfying $ \int_Ωu_0=m$ which are such that for some $T>0$, ($\star$) possesses a uniquely determined classical solution $(u,v)$ in $Ω\times (0,T)$ blowing up at time $T$ in the sense that $\limsup_{t\nearrow T} \|u(\cdot,t)\|_{L^\infty(Ω)}=\infty$.\abs This result is derived by means of a comparison argument applied to the doubly degenerate scalar parabolic equation satisfied by the mass accumulation function associated with ($\star$).