In this expository paper, we describe a sequence of earlier papers presenting applications of a general theorem regarding pointwise estimates for kernels of Neumann series operators ∑j=0∞Tj\documentclass[12pt]{minimal}
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\begin{document}$$\sum _{j=0}^{\infty } T^j$$\end{document}. Here T is an integral operator with a quasi-metric kernel on a measure space (Ω,ω)\documentclass[12pt]{minimal}
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\begin{document}$$(\Omega , \omega )$$\end{document}, with ‖T‖L2(ω)→L2(ω)<1\documentclass[12pt]{minimal}
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\begin{document}$$\Vert T \Vert _{L^2(\omega ) \rightarrow L^2 (\omega )} <1$$\end{document}. Applications are made to the study of non-negative solutions u to the time-independent Schrödinger equation -▵u=qu\documentclass[12pt]{minimal}
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\begin{document}$$- \triangle u = qu$$\end{document} on a domain Ω⊆Rn,n≥3\documentclass[12pt]{minimal}
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\begin{document}$$\Omega \subseteq \mathbb {R}^n, n \ge 3$$\end{document}, with u=f\documentclass[12pt]{minimal}
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\begin{document}$$u = f $$\end{document} on ∂Ω\documentclass[12pt]{minimal}
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\begin{document}$$\partial \Omega $$\end{document}, where q∈Lloc1(Ω)\documentclass[12pt]{minimal}
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\begin{document}$$q \in L^1_{{\textit{loc}}}(\Omega )$$\end{document} and q and f are non-negative. We obtain a balayage condition on the potential q measuring how rapidly q can blow up at ∂Ω\documentclass[12pt]{minimal}
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\begin{document}$$\partial \Omega $$\end{document} and still allow for an almost everywhere finite solution. We also derive bilateral estimates for the Green’s function and Poisson kernel for the Schrödinger operator -▵-q\documentclass[12pt]{minimal}
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\begin{document}$$-\triangle -q$$\end{document} in terms of q and the Green’s function and Poisson kernel for the Laplacian. These results are first described for a C2\documentclass[12pt]{minimal}
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\begin{document}$$C^2$$\end{document} domain. They are later extended to analogues involving the Martin kernel and harmonic measure on a uniform domain.
