Расулов, Т. “Structure of the Numerical Range of a Friedrichs Model: 1D Case With Rank Two Perturbation”.
Ilmiy Nashrlar Markazi (buxdu.Uz)
, vol. 6, no. 6, July 2021, http://journal.buxdu.uz/index.php/journals_buxdu/article/view/3163.