Kuzhel’S. A. “On the Structure of Incoming and Outgoing Subspaces for a Wave Equations in $ℝ^n$”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 51, no. 5, May 1999, pp. 708-12, https://umj.imath.kiev.ua/index.php/umj/article/view/4656.