The main aim of the paper is to study relations between the location of the essential spectrum and exponential decay estimates of the eigenfunctions of systems of elliptic partial differential equations. To solve this problem, we apply the limit operators method. Applications are given to the problem of exponential decay behavior of eigenfunctions of Schrödinger and Dirac operators.

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The paper is devoted to the ${L^{p}}$-theory of boundary integral operators for boundary value problems described by anisotropic Helmholtz operators with variable coefficients in unbounded domains with unbounded smooth boundary. We prove the invertibility of boundary integral operators for Dirichlet and Neumann problems in the Bessel-potential spaces ${H^{s,p}(\partial D)}$, ${p\in(1,\infty)}$, and the Besov spaces ${B_{p,q}^{s}(\partial D)}$, ${p,q\in[1,\infty]}$. We prove also the Fredholmness of the Robin problem in these spaces and give the index formula.