Long time behavior of solutions of a reaction-diffusion equation on unbounded intervals with Robin boundary conditions

Abstract: We study the long time behavior, as $t\to\infty$, of solutions of $$ \left{ \begin{array}{ll} u_t = u_{xx} + f(u), & x>0, \ t >0,\ u(0,t) = b u_x(0,t), & t>0,\ u(x,0) = u_0 (x)\geqslant 0 , & x\geqslant 0, \end{array} \right. $$ where $b\geqslant 0$ and $f$ is an unbalanced bistable nonlinearity. By investigating families of initial data of the type ${ \sigma \phi }_{\sigma >0}$, where $\phi$ belongs to an appropriate class of nonnegative compactly supported functions, we exhibit the sharp threshold between vanishing and spreading. More specifically, there exists some value $\sigma^*$ such that the solution converges uniformly to 0 for any $0 < \sigma < \sigma^*$, and locally uniformly to a positive stationary state for any $ \sigma > \sigma^*$. In the threshold case $\sigma= \sigma^*$, the profile of the solution approaches the symmetrically decreasing ground state with some shift, which may be either finite or infinite. In the latter case, the shift evolves as $C \ln t$ where~$C$ is a positive constant we compute explicitly, so that the solution is traveling with a pulse-like shape albeit with an asymptotically zero speed. Depending on $b$, but also in some cases on the choice of the initial datum, we prove that one or both of the situations may happen.