Berezansky, Y. M. “On Self-Conjugation of Field Operators and Integral Representations of Whiteman Functionals ”. Ukrains’kyi Matematychnyi Zhurnal, vol. 18, no. 3, Apr. 1966, pp. 3-12, https://umj.imath.kiev.ua/index.php/umj/article/view/8732.