Large time behavior for a porous medium equation in a nonhomogeneous medium with critical density
We study the large time behavior of solutions to the porous medium equation in nonhomogeneous media with critical singular density $$ |x|^{-2}\partial_{t}u=\Delta u^m, \quad \hbox{in} \ \real^N\times(0,\infty), $$ where $m>1$ and $N\geq3$. The asymptotic behavior proves to have some interesting and striking properties. We show that there are different asymptotic profiles for the solutions, depending on whether the continuous initial data $u_0$ vanishes at $x=0$ or not. Moreover, when $u_0(0)=0$, we show the convergence towards a profile presenting a discontinuity in form of a shockwave, coming from an unexpected asymptotic simplification to a conservation law, while when $u_0(0)>0$, the limit profile remains continuous. These phenomena illustrate the strong effect of the singularity at $x=0$. We improve the time scale of the convergence in sets avoiding the singularity. On the way, we also study the large-time behavior for a porous medium equation with convection which is interesting for itself.