RabanovychV. I., and SamoilenkoY. S. “Scalar Operators Representable As a Sum of Projectors”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 53, no. 7, July 2001, pp. 939-52, https://umj.imath.kiev.ua/index.php/umj/article/view/4315.