Gerus, Oleg F. “On the Spectra of a Quaternion Singular Integral Operator and Its Components on Spatial Surfaces”. Bulletin de la Société Des Sciences et Des Lettres de Łódź, vol. 70, no. 1, Aug. 2021, pp. 153-67, https://journals.ltn.lodz.pl/Bulletin/article/view/1270.