Kamornikov, S. F., and V. N. Tyutyanov. “On $\sigma$-Subnormal Subgroups of Finite 3’-Groups”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 72, no. 6, June 2020, pp. 806-11, doi:10.37863/umzh.v72i6.1037.