Zolfaghary, Parvaneh. “DESCRIPTION OF ORDER 3 HYPERGROUPS OVER GROUP ARISING FROM DIHEDRAL GROUP $D_9$”. Proceedings of the YSU A: Physical and Mathematical Sciences, vol. 47, no. 2 (231), June 2013, pp. 28-33, https://doi.org/10.46991/PYSUA.2013.47.2.028.