Abstract:
A finite group $G$ is called $\pi$-special if $G=O_{p_1}(G)\times\dots\times O_{p_n}(G)\times O_{\pi'}(G)$, where $\pi=\{p_1,\dots, p_n\}$. We say that a finite group $G$ is semi-$\pi$-special if the normalizer of every non-normal $\pi$-special subgroup of $G$ is $\pi$-special. We prove that if $G$ is not $\pi$-special but $N_G(A)$ is $\pi$-special for every subgroup $A$ of $G$ such that $A$ is either a $\pi'$-group or a $p$-group for some $p\in\pi$, then the following statements hold: (i) $G/F(G)$ is $\pi$-special. Hence$G$has a Hall$\pi'$-subgroup$H$and a soluble Hall$\pi$-subgroup$E$. (ii) If$G$is not$p$-closed for each$p\in\pi$, then: (1) $H$is normal in$G$and$E$is nilpotent. (2) $O_{p_1}(G)\times\dots\times O_{p_n}(G)\times H$is a maximal$\pi$-special subgroup of$G$and every minimal normal subgroup of$G$is contained in$F(G)$.
Keywords:finite group, $\pi$-soluble group, $\pi$-special group, Sylow subgroup, Hall subgroup.