Projection methods for discrete Schrodinger operators

Let $H$ be the discrete Schr"odinger operator $Hu(n):=u(n-1)+u(n+1)+v(n)u(n)$, $u(0)=0$ acting on $l^2({\mathbf{Z}}^{+})$ where the potential $v$ is real-valued and $v(n)\to 0$ as $n\to \mathrm{\infty }$. Let $P$ be the orthogonal projection onto a closed linear subspace $L\subset l^2({\mathbf{Z}}^{+})$. In a recent paper E.B. Davies defines the second order spectrum ${\rm Spec}_2(H,L)$of$H$relativeto$L$asthesetof$z \in {\bf C}$ such that the restriction to $L$ of the operator $P(H-z{)}^{2}P$ is not invertible within the space $L$. The purpose of this article is to investigate properties of ${\rm Spec}_2(H,L)$when$L$ is large but finite dimensional. We explore in particular the connection between this set and the spectrum of $H$. Our main result provides sharp bounds in terms of the potential $v$ for the asymptotic behaviour of ${\mathrm{S}\mathrm{p}\mathrm{e}\mathrm{c}}_2(H,L)$ as $L$ increases towards $l^2({\mathbf{Z}}^{+})$.