Local and global properties of solutions of heat equation with superlinear absorption

We study the limit, when $k\to\infty$ of solutions of $u_t-\Delta u+f(u)=0$ in $R^N\times(0,\infty)$ with initial data k\gd, when $f$ is a positive increasing function. We prove that there exist essentially three types of possible behaviour according $f^{-1}$ and $F^{-1/2}$ belong or not to $L^1(1,\infty)$, where $F(t)=\int_0^t f(s)ds$. We emphasize the case where f(u)=u((\ln u+1))^{\alpha}. We use these results for giving a general result on the existence of the initial trace and some non-uniqueness results for regular solutions with unbounded initial data

Initial trace of positive solutions of a class of degenerate heat equation with absorption

We study the initial value problem with unbounded nonnegative functions or measures for the equation \prt_tu-\Gd_p u+f(u)=0 in \BBR^N\ti(0,\infty) where $p>1$, \Gd_p u = \text{div}(\abs {\nabla u}^{p-2} \nabla u) and $f$ is a continuous, nondecreasing nonnegative function such that $f(0)=0$. In the case $p>\frac{2N}{N+1}$, we provide a sufficient condition on $f$ for existence and uniqueness of the solutions satisfying the initial data k\gd_0 and we study their limit when $k\to\infty$ according $f^{-1}$ and $F^{-1/p}$ are integrable or not at infinity, where F(s)=\int_0^s f(\gs)d\gs. We also give new results dealing with non uniqueness for the initial value problem with unbounded initial data. If $p>2$, we prove that, for a large class of nonlinearities $f$, any positive solution admits an initial trace in the class of positive Borel measures. As a model case we consider the case f(u)=u^\ga \ln^\gb(u+1), where \ga>0 and \gb\geq 0