Gubreev G. M., and Ignatenko T. R. “On the Class of Orthogonalizators of the Families of Exponents With Real Frequencies”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 44, no. 8, Sept. 1992, pp. 1031-44, https://umj.imath.kiev.ua/index.php/umj/article/view/8031.