[1] R. Brandl , Groups with few non-normal subgroups, Comm. Algebra, 23 (1995) 2091-2098.
[2] R. Brandl , Conjugacy classes of non-normal subgroups of nite p-groups, Israel J. of Math., 195 (2013) 473-479.
[3] R. Dedekind , Ub er Grupp en deren samtliche teiler normalteiler sind, Math. Ann., 48 (1897) 548-561.
[4] G. A. Fernandez-Alcober and L. Legarreta , The nite p -groups with p conjugacy classes of non-normal sub-
groups, Israel J. of Math., 180 (2010) 189-192.
[5] D. Gorenstein, Fnite Groups, Harp er and Row, New York, 1980.
[6] L. Gong , H. Cao and G. Chen , Finite nilp otent groups having exactly four conjugacy classes of non-normal subgroups, Algebra Col loq., 20 (2013) 579-592.
[7] C. Yin Ho , Pro jective planes with a regular collineation group and a question ab out p owers of a prime, J. Algrbra, 154 (1993) 141-151.
[8] J. Lu and W. Meng , On solvability of nite groups with few non-normal subgroups, Comm. Algebra, 1 (2015) 1752-1756.
[9] H. Mousavi , On nite groups with few non-normal subgroups, Comm. Algebra, 27 (1999) 3143-3151.
[10] H. Mousavi , Nilp otent groups with three conjugacy classes of non-normal subgroups, Bul letin Iranian Math. Soc., 40 (2014) 1291-1300.
[11] H. Mousavi , Non-nilp otent groups with three conjugacy classes of non-normal subgroups, Int. J. Group Theory, 3 (2014) 1-7.
[12] D. Oggionni, G. Ponzoni and V. Zambelli , Groups with few non-normal cyclic subgroups, J. Note Math., 30 (2010) 121-133.
[13] Shirong Li , The numb er of conjugacy classes of non-normal cyclic subgroups in nilpotent groups of odd order, J. Group Theory, vv (1998) 165-171.