Kuzhel’, S. A., and O. M. Patsyuk. “On the Theory of $\mathcal{PT}$-Symmetric Operatorss”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 64, no. 1, Jan. 2012, pp. 32-49, https://umj.imath.kiev.ua/index.php/umj/article/view/2554.