Eternal solutions to a porous medium equation with strong non-homogeneous absorption. Part I: radially non-increasing profiles

Existence of specific eternal solutions in exponential self-similar form to the following quasilinear diffusion equation with strong absorption\[ \partial_t u=\Delta u^m-|x|^{\sigma}u^q, \]posed for $(t,\,x)\in (0,\,\infty )\times \mathbb {R}^N$, with $m>1$, $q\in (0,\,1)$ and $\sigma =\sigma _c:=2(1-q)/ (m-1)$ is proved. Looking for radially symmetric solutions of the form\[ u(t,x)={\rm e}^{-\alpha t}f(|x|\,{\rm e}^{\beta t}), \quad \alpha=\frac{2}{m-1}\beta, \]we show that there exists a unique exponent $\beta ^*\in (0,\,\infty )$ for which there exists a one-parameter family $(u_A)_{A>0}$ of solutions with compactly supported and non-increasing profiles $(f_A)_{A>0}$ satisfying $f_A(0)=A$ and $f_A'(0)=0$. An important feature of these solutions is that they are bounded and do not vanish in finite time, a phenomenon which is known to take place for all non-negative bounded solutions when $\sigma \in (0,\,\sigma _c)$.