1. S. Adnan, On groups having exactly 2 conjugacy classes of maximal subgroups, Lincei-Rend. Sc. fis. mat. e net, 66 (1979), 175–178.
2. Z. M. Chen, On inner-p-closed groups, (in Chinese) Adv. Math., 15:4 (1986), 385–388.
3. Z. M. Chen and Y. M. Wang, Minimal non-solvable groups with an acting group, (in Chinese) Koxue Tong Bao, 34:22 (1989), 1691–1693.
4. X. Y. Guo, The normal index of maximal subgroups in finite groups, (in Chinese) Acta Math. Sin., 34 (1991), 208–212.
5. P. Hall, J. Math., 182 (1940), 206–214.