1. [1] S. Agmon, Lower bounds for solutions of Schrödinger equations, J. Analyse Math. 23 (1970), 1–25.
2. [2] S. Agmon and L. Hörmander, Asymptotic properties of solutions of differential equations with simple characteristics, J. Analyse Math. 30 (1976), 1–38.
3. [3] M. S. P. Eastham and H. Kalf, Schrödinger-type operators with continuous spectra, Research Notes in Mathematics, vol. 65, Pitman (Advanced Publishing Program), Boston, Mass., 1982.
4. [4] D. M. Eidus, The principle of limit amplitude, Russian Math. Surveys 24 (1969), 97–167.
5. [5] R. Froese and I. Herbst, Exponential bounds and absence of positive eigenvalues for $N$-body Schrödinger operators, Comm. Math. Phys. 87 (1982/83), no. 3, 429–447.