Skeleton Integral Equations for Acoustic Transmission Problems with Varying Coefficients

In this paper we will derive an integral equation which transform a three-dimensional acoustic transmission problem with \textit{variable} coefficients, non-zero absorption, and mixed boundary conditions to a non-local equation on the skeleton of the domain $\Omega\subset\mathbb{R}^{3}$, where ``skeleton'' stands for the union of the interfaces and boundaries of a Lipschitz partition of $\Omega$. To that end, we introduce and analyze abstract layer potentials as solutions of auxiliary coercive full space variational problems and derive jump conditions across domain interfaces. This allows us to formulate the non-local skeleton equation as a \textit{direct method} for the unknown Cauchy data of the original partial differential equation. We establish coercivity and continuity of the variational form of the skeleton equation without based on an auxiliary full space variational problem. Explicit knowledge of Green's functions is not required and our estimates are explicit in the complex wave number.