The eternal solutions of parabolic equations with boundary condition
Jingqi Liang, Lidan Wang
TL;DR
This work analyzes parabolic equations on unbounded cylinders with zero lateral boundary, focusing on the space of positive solutions under a uniform lower bound. For the homogeneous case ($f=0$), the authors prove that the positive solution set is one-dimensional and exhibits exponential growth/decay at ±∞; for the inhomogeneous case ($f\neq0$), every solution is the sum of a bounded particular solution and a homogeneous solution, yielding a precise affine structure of the full solution set. The key tools are a robust maximum principle and Harnack-type inequalities, enabling a detailed description of asymptotics and a sharp decomposition of solutions. The results extend elliptic-type structure theorems to parabolic equations on unbounded domains and provide a framework for understanding bounded-below solutions in the presence of lower-order terms.
Abstract
In this paper, we study the parabolic equations of the form $$ \left\{ \begin{array}{rcll} Lu(y,t) &=& f, \qquad &(y,t)\in Q,\\ u(y,t)&=& 0, \qquad &(y,t)\in \partial Q, \\ u(y,t)&& \hspace{-8mm}\mbox{is uniformly bounded from below}, \qquad &(y,t)\in Q, \end{array} \right. $$ where $Q=Ω\times\mathbb{R}\subset\mathbb{R}^{n+1}$ and $Ω\subset\mathbb{R}^{n}$ is a bounded Lipschitz domain with $0\inΩ$. Here $L$ is a general second order uniformly parabolic differential operator in non-divergence form or divergence form. For $f=0$, we establish the structure of the solution space, which is one dimensional and the solutions in this space grow exponentially at one end and decay exponentially at the other. For $f\neq0$, we show that all solutions can be presented by the solutions corresponding to the homogenous equations($f=0$) and a bounded special solution of the inhomogeneous equations. Our method is based on maximum principle in $Q$ and the Harnack type inequalities.