Vladimir Georgescu, Andrei Iftimovici Localizations at infinity and essential spectrum of quantum Hamiltonians: I. General theory (726K, postscript) ABSTRACT. We isolate a large class of self-adjoint operators $H$ in $L^2(X)$ whose essential spectrum is determined by their behavior at $x\sim\infty$ and we give a canonical representation of the essential spectrum of $H$ in terms of spectra of limits at infinity of translations of $H$. The configuration space $X$ is an abelian locally compact not compact group.