Nudel'man, M. A. “On Initial Data of a Simple Conservative Scattering System That Can Be Transferred to Zero by a Sequence of Inputs from L ²”. Ukrains’kyi Matematychnyi Zhurnal, vol. 57, no. 4, Apr. 2005, pp. 559–563, https://umj.imath.kiev.ua/index.php/umj/article/view/3621.