Kapitanski, L., I. Rodnianski, and K. Yajima. “On the Fundamental Solution of a Perturbed Harmonic Sscillator”. Topological Methods in Nonlinear Analysis, vol. 9, no. 1, Mar. 1997, pp. 77-106, https://apcz.umk.pl/TMNA/article/view/TMNA.1997.005.