P. . Zolfaghary, “DESCRIPTION OF ORDER 3 HYPERGROUPS OVER GROUP ARISING FROM DIHEDRAL GROUP $D_9$”, Proc. YSU A: Phys. Math. Sci., vol. 47, no. 2 (231), pp. 28–33, Jun. 2013, doi: 10.46991/PYSUA.2013.47.2.028.