Rabanovych, V. I. “On the Decomposition of a Diagonal Operator into a Linear Combination of Idempotents or Projectors”. Ukrains’kyi Matematychnyi Zhurnal, vol. 57, no. 3, Mar. 2005, pp. 388–393, https://umj.imath.kiev.ua/index.php/umj/article/view/3606.