Existence results of two mixed boundary value elliptic PDEs in $\mathbb{R}^n$

Abstract: We study the existence of a solution to the mixed boundary value problem for Helmholtz and Poisson type equations in a bounded Lipschitz domain $\Omega\subset\mathbb{R}^N$ and in $\mathbb{R}^N\setminus\Omega$ for $N\geq3$. The boundary $\partial\Omega$ of $\Omega$ is the decomposition of $\Gamma_1,\Gamma_2\subset\partial\Omega$ such that $\partial\Omega=\Gamma=\overline{\Gamma}_1\cup\Gamma_2=\Gamma_1\cup\overline{\Gamma}_2$ and $\Gamma_1\cap\Gamma_2=\emptyset$. We have shown that if the Neumann data $f_2\in H^{-\frac{1}{2}}(\Gamma_2)$ and the Dirichlet data $f_1\in H^{\frac{1}{2}}(\Gamma_1)$ then the Helmholtz problem with mixed boundary data admits a unique solution. We have also shown the existence of a weak solution to a mixed boundary value problem governed by the Poisson equation with a measure data and the Dirichlet, Neumann data belongs to $f_1\in H^{\frac{1}{2}}(\Gamma_1)$, $f_2\in H^{-\frac{1}{2}}(\Gamma_2)$ respectively.