  We classify the blow up self-similar profiles for the following
reaction-diffusion equation with weighted reaction $$ u_t=(u^m)_{xx} +
|x|^{\sigma}u^m, $$ posed for $(x,t)\in\real\times(0,T)$, with $m>1$ and
$\sigma>0$. In strong contrast with the well-studied equation without the
weight (that is $\sigma=0$), on the one hand we show that for $\sigma>0$
sufficiently small there exist \emph{multiple self-similar profiles with
interface} at a finite point, more precisely, given any positive integer $k$,
there exists $\delta_k>0$ such that for $\sigma\in(0,\delta_k)$, there are at
least $k$ different blow up profiles with compact support and interface at a
positive point. On the other hand, we also show that for $\sigma$ sufficiently
large, the blow up self-similar profiles with interface \emph{cease to exist}.
This unexpected balance between existence of multiple solutions and
non-existence of any, when $\sigma>0$ increases, is due to the effect of the
presence of the weight $|x|^{\sigma}$, whose influence is the main goal of our
study. We also show that for any $\sigma>0$, there are no blow up profiles
supported in the whole space, that is with $u(x,t)>0$ for any $x\in\real$ and
$t\in(0,T)$.

Analysis of PDEs