Berezansky Y. М., and Konstantinov A. Y. “Multiparametric Spectral Problems As Expanded by the author’s Own Vectors ”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 44, no. 7, Aug. 1992, pp. 901-13, https://umj.imath.kiev.ua/index.php/umj/article/view/8122.