1. Fuzzy groups redefined;Anthony;J. Math. Anal. Appl.,1979
2. Joint spaces, power sets, fuzzy sets;Corsini,1994
3. Prolegomena of Hypergroup;Corsini,1993
4. From groupoids to groups through hypergroups;Corsini;Rendiconti Math. S.,1989
5. Fuzzy groups and level subgroups;Das;J. Math. Anal. Appl.,1981