SkibaA. N., and ZhangC. “On $\Sigma_t^{σ}$ -Closed Classes of Finite Groups”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 70, no. 12, Dec. 2018, pp. 1707-16, https://umj.imath.kiev.ua/index.php/umj/article/view/1669.