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${\mathbb {R}^N}$Published online by Cambridge University Press: 10 October 2025
We consider the asymptotics of long-time behavior of a solution u of the semilinear parabolic problem
$\partial _tu=\Delta u-u+u|u|^{p-2}$ in
${\mathbb {R}^N}\times (0,\infty )$,
$u(0)=u_0\in H^1({\mathbb {R}^N})\cap L^\infty ({\mathbb {R}^N})$. Since the spatial domain on which the problem is posed is noncompact, we cannot expect the relative compactness of the solution orbit, e.g., in
$H^1({\mathbb {R}^N})$ in general. In this article, we prove that the compactness of the orbit holds up to the ground state energy level, namely, if
$\lim _{t\to \infty }I(u(t))\leq d_\infty $, where I is the energy functional associated with (P) and
$d_\infty $ its ground state energy, then the orbit of
$u(t)$ is compact in
$H^1({\mathbb {R}^N})$. Our result includes the previous results in [4, 5].
This work was supported by JSPS KAKENHI Grant Numbers JP19H05599. The second author is supported by JSPS KAKENHI Grant-in-Aid for JSPS Fellows 23KJ1416.