Kuzhel', S. A., and L. Vavrykovych. “On Infinite-Rank Singular Perturbations of the Schrödinger Operator”.
Ukrains’kyi Matematychnyi Zhurnal
, vol. 60, no. 4, Apr. 2008, pp. 487–496,
https://umj.imath.kiev.ua/index.php/umj/article/view/3171
.