Regularity of almost periodic solutions of Poisson equation

This paper discusses some regularity of almost periodic solutions of the Poisson equation $-\Delta u = f$ in $\mathbb{R}^n$, where $f$ is an almost periodic function. It wasproved by Sibuya [Almost periodic solutions of Poisson's equation. Proc. Amer. Math. Soc., 28:195–198, 1971.] that if $u$ is a bounded continuous function and solves the Poisson equation in the distribution sense, then $u$ is an almost periodic function. In this work, we weaken the assumption of the usual boundedness to boundedness in the sense of distribution, which we refer to as a bounded generalized function. The set of bounded generalized functions are wider than the set of usual bounded functions. Then, assuming that $u$ is a bounded generalized function and solves the Poisson equation in the distribution sense, we prove that this solution is bounded in the usual sense, continuous and almost periodic. Moreover, we show that the first partial derivatives of the solution $\partial u/ \partial x_i$, $i=1, \ldots, n$, are also continuous, bounded and almost periodic functions. The technique is based on extending a representation formula using Green function for Poisson equation for solutions in the distribution sense. Some useful properties of distributions are also shown that can be used in studying other elliptic problems.