Agmon's type estimates of exponential behavior of solutions of systems of elliptic partial differential equations.Applications to Schr\"{o}dinger, Moisil-Theodorescu and Dirac operators

The aim of this paper is to derive Agmon's type exponential estimates for solutions of elliptic systems of partial differential equations on $\sR^n$. We show that these estimates are related with the essential spectra of a family of associated differential operators which depend on the original operator, and with exponential weights which describe the decrease of solutions at infinity. The essential spectra of the involved operators are described by means of their limit operators. The obtained results are applied to study the problem of exponential decay of eigenfunctions of matrix Schr\"{o}dinger, Moisil-Theodorescu, and Dirac operators.